This allows applications to start a Flutter engine instance during app startup
and keep it running throughout the app process' lifetime.
FlutterActivity subclasses can override createFlutterNativeView to provide a
preinitialized FlutterNativeView instance and override retainFlutterNativeView
to signal that the FlutterNativeView should be kept alive when the activity
is destroyed.
This creates a `FlutterFragmentActivity` class that extends
the Android v4 Support librray's `FragmentActivity` class.
However, we intentionally do not bundle the support library
with our engine, so apps that wish to use this class are
responsible for including the support library .jar file in
their runtime deps when creating the final app.
flutter/flutter#10072
