FlutterFragment should support owning the engine across configuration change

[xster]

Currently, the FlutterFragment can create and own its own engine or have one injected into it.

The FlutterFragment creating its own engine can have a problem when the engine should outlive the instance of the FlutterFragment. This can happen on configuration change (https://developer.android.com/guide/topics/resources/runtime-changes) such as rotating the screen. When the FlutterFragment is destroyed and recreating with the new configuration, the same FlutterEngine instance should be preserved.

This isn't a problem generally because we instruct Android to not change configuration on the FlutterActivity

flutter/packages/flutter_tools/templates/app_shared/android.tmpl/app/src/main/AndroidManifest.xml.tmpl

Line 12 in 361d416

android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"

. But this isn't possible in hybrid apps where native UI also exist alongside the FlutterFragment. In that case, the native UI relies on the other fragments in the activity being destroyed and recreated to adjust to the new configuration. But the FlutterFragment should be destroyed and recreated with the FlutterEngine it owns also being destroyed and recreated.

This can be done either by temporarily saving the owned FlutterEngine instance in the FlutterEngineCache or by using Android's https://developer.android.com/reference/androidx/lifecycle/ViewModel mechanism, as long as that AndroidX mechanism is already part of the Flutter engine https://github.com/flutter/engine/blob/master/tools/androidx/files.json#L3 and we're not paying more size cost for it.