This factors the functionality that was in `FlutterActivity`
to live in `FlutterActivityDelegate`. This will allow the creation of a
`FlutterFragmentActivity` that has the same core functionality, which in
turn unlocks certain Android plugins that choose to require the v4
support library (like Google Sign-In).
https://github.com/flutter/flutter/issues/10072
FragmentActivity is required for plugins that use auto managed Google APIs, such as Google Sign-In. Requires flutter/flutter#9036 to land so that gradle builds include FragmentActivity.
