ANRs in `SyncService.start/stop`

We recently became aware of an abnormal growth of ANR (application not responding) issues, and the most common one seems to be one with the next stacktrace:

Stacktrace

  #00  pc 0x000000000004dc5c  /apex/com.android.runtime/lib64/bionic/libc.so (syscall+28)

  #01  pc 0x000000000381f610  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk

  #02  pc 0x00000000028a5f00  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk

  #03  pc 0x0000000001b2897c  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk (ffi_matrix_sdk_ffi_rust_future_poll_u32+124)

  #04  pc 0x000000000001404c  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk (BuildId: ddeb5c7dcafc8a4ed4ba180f840e1967d8207a8b)

  #05  pc 0x0000000000010a78  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk (BuildId: ddeb5c7dcafc8a4ed4ba180f840e1967d8207a8b)

  #06  pc 0x00000000000063c8  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk (BuildId: ddeb5c7dcafc8a4ed4ba180f840e1967d8207a8b)

  #07  pc 0x00000000000082dc  /data/app/~~kNW33dJeKDR24qAYlClfVw==/io.element.android.x-PLPFGHZP_N-S7DxV4AkKdQ==/split_config.arm64_v8a.apk (Java_com_sun_jna_Native_invokeVoid+32) (BuildId: ddeb5c7dcafc8a4ed4ba180f840e1967d8207a8b)

  at com.sun.jna.Native.invokeVoid (Native.java)

  at com.sun.jna.Function.invoke (Function.java:418)

  at com.sun.jna.Function.invoke (Function.java:364)

  at com.sun.jna.Library$Handler.invoke (Library.java:270)

  at java.lang.reflect.Proxy.invoke (Proxy.java:1006)

  at org.matrix.rustcomponents.sdk.UniffiLib.ffi_matrix_sdk_ffi_rust_future_poll_void (matrix_sdk_ffi.kt)

  at org.matrix.rustcomponents.sdk.SyncService$start$3.invoke (SyncService.java:20015)

  at org.matrix.rustcomponents.sdk.SyncService$start$3.invoke (SyncService.java:20008)

  at org.matrix.rustcomponents.sdk.Matrix_sdk_ffiKt.uniffiRustCallAsync (matrix_sdk_ffi.kt:5532)

  at org.matrix.rustcomponents.sdk.Matrix_sdk_ffiKt$uniffiRustCallAsync$1.invokeSuspend (matrix_sdk_ffi.kt:17)

  at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith (ContinuationImpl.kt:33)

  at kotlinx.coroutines.DispatchedTaskKt.resume (DispatchedTask.kt:221)

  at kotlinx.coroutines.CancellableContinuationImpl.cancelCompletedResult$kotlinx_coroutines_core (CancellableContinuationImpl.kt:177)

  at kotlinx.coroutines.DispatchedTaskKt.resumeUnconfined (DispatchedTask.kt:177)

  at kotlinx.coroutines.DispatchedTaskKt.dispatch (DispatchedTask.kt:149)

  at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume (CancellableContinuationImpl.kt:470)

  at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$kotlinx_coroutines_core (CancellableContinuationImpl.kt:504)

  at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$kotlinx_coroutines_core$default (CancellableContinuationImpl.kt:493)

  at kotlinx.coroutines.CancellableContinuationImpl.resumeWith (CancellableContinuationImpl.kt:359)

  at org.matrix.rustcomponents.sdk.uniffiRustFutureContinuationCallbackImpl.callback (matrix_sdk_ffi.kt:5517)

  at java.lang.reflect.Method.invoke (Native method)

  at com.sun.jna.CallbackReference$DefaultCallbackProxy.invokeCallback (CallbackReference.java:585)

  at com.sun.jna.CallbackReference$DefaultCallbackProxy.callback (CallbackReference.java:613)

  at com.sun.jna.Native.invokeVoid (Native.java)

  at com.sun.jna.Function.invoke (Function.java:418)

  at com.sun.jna.Function.invoke (Function.java:364)

  at com.sun.jna.Library$Handler.invoke (Library.java:270)

  at java.lang.reflect.Proxy.invoke (Proxy.java:1006)

  at org.matrix.rustcomponents.sdk.UniffiLib.ffi_matrix_sdk_ffi_rust_future_poll_void (matrix_sdk_ffi.kt)

  at org.matrix.rustcomponents.sdk.SyncService$stop$3.invoke (SyncService.java:20049)

  at org.matrix.rustcomponents.sdk.SyncService$stop$3.invoke (SyncService.java:20042)

  at org.matrix.rustcomponents.sdk.Matrix_sdk_ffiKt.uniffiRustCallAsync (matrix_sdk_ffi.kt:5532)

  at org.matrix.rustcomponents.sdk.Matrix_sdk_ffiKt$uniffiRustCallAsync$1.invokeSuspend (matrix_sdk_ffi.kt:17)

  at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith (ContinuationImpl.kt:33)

  at kotlinx.coroutines.DispatchedTask.run (DispatchedTask.kt:101)

  at android.os.Handler.handleCallback (Handler.java:938)

  at android.os.Handler.dispatchMessage (Handler.java:99)

  at android.os.Looper.loopOnce (Looper.java:346)

  at android.os.Looper.loop (Looper.java:475)

  at android.app.ActivityThread.main (ActivityThread.java:7889)

  at java.lang.reflect.Method.invoke (Native method)

  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:548)

  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1009)

Which points to a `SyncService.stop()` call being shortly followed by a `SyncService.start()` one. This only happens at 3 points in the app:

- In [LoggedInFlowNode.observeSyncStateAndNetworkStatus()](https://github.com/element-hq/element-x-android/blob/97f3be3dc5352aea69d3568343f417fdbd59c8ef/appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowNode.kt#L190), which will stat the sync service both when the main activity of the app starts (think of it as the main navigation flow) and when the network is available if the service wasn't already running. The service will be stopped when the activity is no longer active too (as in, visible to the user).
- In [CallScreenPresenter.HandleMatrixClientSyncState()](https://github.com/element-hq/element-x-android/blob/97f3be3dc5352aea69d3568343f417fdbd59c8ef/features/call/impl/src/main/kotlin/io/element/android/features/call/impl/ui/CallScreenPresenter.kt#L229): the Element Call screen runs on a different activity so the callbacks in `LoggedInFlowNode` won't start the sync service when needed and we need to manually do it here. When the call is finished and the screen removed, the service will be stopped.
- In [SyncOnNotifiableEvent.startSyncIfNeeded()/stopSyncIfNeeded()](https://github.com/element-hq/element-x-android/blob/97f3be3dc5352aea69d3568343f417fdbd59c8ef/libraries/push/impl/src/main/kotlin/io/element/android/libraries/push/impl/push/SyncOnNotifiableEvent.kt#L85): when we receive a push notification we start an opportunistic sync, then wait until either the user joins the call if it was a ringing call notification or we receive the event related to the notification in any other case, or a timeout happens, then stops the service again unless the app is now visible again.

The implementation of the flows above aren't perfect, and we may be calling `SyncService.start/stop` more than needed, which may be one of the root causes of this issue... centralising all these calls in a single component would probably make more sense.

The calls to these methods are all done in a separate coroutine, but all of them running on the `Main` dispatcher, which means if the underlying thread gets blocked somehow, the whole app freezes since it's the same thread that runs the UI. While this *shouldn't* happen because UniFFI seems to translate `async` methods to suspending functions, it seems to be happening somehow.

I wasn't able to reproduce this issue without a little help: I had to artificially add some delay inside `SyncService.start()` on the SDK so the app froze when I foregrounded/backgrounded it several times, but I imagine the real env would be similar. With these changes to force more frequent app freezes, the issue could be mitigated by running the `SyncService.stop()` calls in a background dispatcher (surprisingly, doing the same for `SyncService.start()` had no effect AFAICT): I'm not sure if this meant we now had a blocked background thread or if by being on background this meant the apparent deadlock we saw before just couldn't happen.

As for what might have caused this, the issue apparently started at [v0.7.6](https://github.com/element-hq/element-x-android/releases/tag/v0.7.6), and the related SDK changes are [these](https://github.com/matrix-org/matrix-rust-sdk/compare/ee93c278dff6f343b322bbf2a8be4115cfb30b40...matrix-sdk-0.9.0) if I'm not mistaken. Nothing seems really related to this other than [this PR](https://github.com/element-hq/element-x-android/pull/4069) in EXA but that doesn't seem to be the cause, so maybe it's something coming from a UniFFI internal change.