Allow Flutter focus to interop with Android view hierarchies

[blasten]

When platform views are used in the Flutter widget tree, the keyboard may be dismissed or shown incorrectly.

The issue was present in hybrid composition too. When HC is used, this issue can be solved by checking
if any descendant view of the current platform view has been focused.

Fixes flutter/flutter#81029
Fixes flutter/flutter#34169 for HC.

This PR will also close flutter/flutter#34169 since using HC is the solution.

screen-20210605-120831_2.mp4

[GaryQian]

Is HC meant to stand for "hierarchy"? Not sure if this is a standard abbreviation, but it was not intuitive for me. It may be clearer to just write it out.

[blasten]

I clarified it in the comment above.

[GaryQian]

LGTM % a few nits.

[math1man]

FYI, we believe this change is causing thousands of crashes on our app. You cannot register a ViewTreeObserver.OnGlobalFocusChangeListener without also unregistering it when you're done with it. ViewTreeObserver listeners are global to the view hierarchy and unless unregistered will persist even after the view that registered them is destroyed. At best, this leaks the view, and at worst, causes crashes like this:

java.lang.NullPointerException
	at io.flutter.plugin.platform.PlatformViewsController.lambda$initializePlatformViewIfNeeded$0$PlatformViewsController(PlatformViewsController.java:746)
	at io.flutter.plugin.platform.PlatformViewsController$$ExternalSyntheticLambda0.onFocusChange(D8$$SyntheticClass)
	at io.flutter.embedding.engine.mutatorsstack.FlutterMutatorView$1.onGlobalFocusChanged(FlutterMutatorView.java:91)
	at android.view.ViewTreeObserver.dispatchOnGlobalFocusChange(ViewTreeObserver.java:960)
	at android.view.View.handleFocusGainInternal(View.java:6695)
	at android.view.ViewGroup.handleFocusGainInternal(ViewGroup.java:720)
	at android.view.View.requestFocusNoSearch(View.java:9920)
	at android.view.View.requestFocus(View.java:9899)
	at android.view.ViewGroup.requestFocus(ViewGroup.java:3048)
	at android.view.View.requestFocus(View.java:9866)
	at android.view.View.requestFocus(View.java:9845)
	at android.view.ViewRootImpl.focusableViewAvailable(ViewRootImpl.java:3724)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:763)
	at android.view.View.setFlags(View.java:12775)
	at android.view.View.setVisibility(View.java:8814)
	<14 redacted stack trace lines>
	at android.os.Handler.handleCallback(Handler.java:751)
	at android.os.Handler.dispatchMessage(Handler.java:95)
	at android.os.Looper.loop(Looper.java:154)
	at android.app.ActivityThread.main(ActivityThread.java:6816)
	at java.lang.reflect.Method.invoke(Method.java:-2)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1563)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1451)

We have a short-term mitigation, but the issue needs to be fixed ASAP.

[blasten]

@math1man thanks. working on the fix right now

[math1man]

Sweet