firebase::Mutex::Acquire ANR after adding FCM and Firestore SDK

Hello. After we added Firebase Messaging and Firestore SDK to the app, we have some ANR from firebase::Mutex::Acquire.

Firebase SDKs are all of version 12.10.0, and Unity is of version 6000.0.65f1.

```
main (native):tid=1 systid=9675
#00 pc 0x57840 libc.so (syscall + 32) (BuildId: a2e858dbd9a5ee8fabb561b121af2989)
#01 pc 0x5c4c0 libc.so (__futex_wait_ex + 148) (BuildId: a2e858dbd9a5ee8fabb561b121af2989)
#02 pc 0xc4a78 libc.so (NonPI::MutexLockWithTimeout + 604) (BuildId: a2e858dbd9a5ee8fabb561b121af2989)
#03 pc 0x19f314 split_config.arm64_v8a.apk (firebase::Mutex::Acquire + 8)
#04 pc 0x2cd3d8 split_config.arm64_v8a.apk + 81920
#05 pc 0x1a9fe0 split_config.arm64_v8a.apk (firebase::ReferenceCountedFutureImpl::ReleaseMutexAndRunCallbacks + 392)
#06 pc 0x2480cc split_config.arm64_v8a.apk (void firebase::ReferenceCountedFutureImpl::CompleteInternal, std::__ndk1::allocator>, void firebase::ReferenceCountedFutureImpl::CompleteWithResultInternal, std::__ndk1::allocator>>::'lambda'> + 216)
#07 pc 0x2ccc34 split_config.arm64_v8a.apk + 81920
#08 pc 0x194814 split_config.arm64_v8a.apk (firebase::util::JniResultCallback_nativeOnResult + 232)
at com.google.firebase.app.internal.cpp.JniResultCallback.nativeOnResult(Native method)
at com.google.firebase.app.internal.cpp.JniResultCallback.onCompletion(JniResultCallback.java:154)
at com.google.firebase.app.internal.cpp.JniResultCallback$TaskCallback.onSuccess(JniResultCallback.java:70)
at com.google.android.gms.tasks.zzm.run(com.google.android.gms:play-services-tasks@@18.2.1:1)
at android.os.Handler.handleCallback(Handler.java:1000)
at android.os.Handler.dispatchMessage(Handler.java:104)
at android.os.Looper.loopOnce(Looper.java:242)
at android.os.Looper.loop(Looper.java:362)
at android.app.ActivityThread.main(ActivityThread.java:8448)
at java.lang.reflect.Method.invoke(Native method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:992)
```