Add a FlutterEngine cleanup method to FlutterActivity and FlutterFragment.

There is an existing method called configureFlutterEngine(engine) where developers register plugins and do any other configuration they want to a FlutterEngine. This method is invoked from from onCreate() in FlutterActivity and onAttach() in FlutterFragment. This makes it difficult to effectively document the correct place to clean up FlutterEngine references, e.g, a channel that is no longer needed.

To remedy this, we should introduce something like a teardownFlutterEngine(engine) method that gives FlutterActivity and FlutterFragment an explicit opportunity to clean up references that were established in configureFlutterEngine(engine).