This PR adds 2 features to RenderAndroidView and AndroidView:

1. Hit testing behavior
Adds a `PlatformViewHitTestBehavior` which is similar to
`HitTestBehavior` without the `deferToChild` option (as platform views
don't have child render objects) and with a `transparent` option which
prevents it from forwarding any events to the Android view.

2. MotionEvent recomposing logic
FlutterView and the framework `converter.dart` are working together to
transform each Android MotionEvent object into one or more
`PointerEvent` objects.

This PR adds the reverse logic (in _MotionEventDispatcher which is used
by RenderAndroidView) which turns a stream of PointerEvent objects into
MotionEvent objects.

The correctness of the recomposing logic is tested in an integration
test which will land in a separate PR (the unit test PR is pretty big,
trying to keep as many bite-size PRs for reviewer's convenience)