Sometimes, plugin instances need access to the `FlutterView`.
They can currently cast the `messenger()`, but that's fragile.
This adds API support for getting the view from the registry.
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
Sarah Zakarias
Mikkel Nygaard Ravn
Jason Simmons
Ali Bitek
Zachary Anderson
Chinmay Garde
Todd Volkert