Resizing an AndroidView happens asynchronously (as it requires thread
hopping from the ui thread to the platform thread).
While waiting for the resize to complete the framework does exactly when
the embedded view has been resized. This leads to pretty bad jank as the
framework might paint the embedded view with a wrong size.

We're working around this by freezing the texture frame while resizing
until we are sure that the next frame has the new size. This is how it
looks with the workaround:

This is how it looks before and after the workaround:

Before (janky) |  After (less janky)
:--------|---------:
![resize_before](https://user-images.githubusercontent.com/1024117/43669639-51bfd0ae-9739-11e8-8cbe-96e36f2460d2.gif) | ![resize_after](https://user-images.githubusercontent.com/1024117/43669647-62e885a6-9739-11e8-8aa4-beb74e979995.gif)

This depends on flutter/engine#5938. Additionaly right now the engine completes
the resize call immediately, a following PR will change it to complete
after a frame with the new size is ready.

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)

RenderAndroidView is responsible for sizing and displaying an embedded
Android view.
AndroidView is responsible for creating and disposing the Android view
and is using RenderAndroidView to display it.