Introduction of the GestureHandler interface

The GestureHandler interface allows us to simulate gesture events
(scroll and drag) generated by Compose. This is useful for testing the
behavior of our components, as we can verify that they are responding to
 gestures as expected.

Test: atest SceneGestureHandlerTest
Bug: 291025415
Change-Id: If8afda7f7ea02703cc6829c1a967a616dfa09859