Incorrect behavior of app background state

Not something I experienced while using the app, but something I noticed while having a look at the source code.

The `mIsInBackground` state in `Simplenote.java` is set to false when an `onTrimMemory()` event other than `TRIM_MEMORY_UI_HIDDEN` happens. See: https://github.com/Automattic/simplenote-android/blob/develop/Simplenote/src/main/java/com/automattic/simplenote/Simplenote.java#L338

I believe this is incorrect. `onTrimMemory()` can be called with other values for `level` while the app is in the background, so then `mIsInBackground` will be incorrectly set to false. See: https://developer.android.com/reference/android/content/ComponentCallbacks2

A quick fix could be to remove the `else` of that `if` statement. But maybe using ProcessLifecycleOwner instead of ComponentCallbacks2 would be a more suited solution: https://developer.android.com/reference/androidx/lifecycle/ProcessLifecycleOwner

Start in Simplenote/src/main/java/com/automattic/simplenote/Simplenote.java around the onTrimMemory() handler at line 338, and compare its state transitions with the ComponentCallbacks2 documentation. Determine how background state should behave for memory levels other than TRIM_MEMORY_UI_HIDDEN, then verify that mIsInBackground remains accurate while the app is backgrounded.

Written by the indexing model from the issue text.