SceneGestureHandler simplification: remove overscroll method

In order to simplify the reasoning about the states of the transition
between scenes, it is possible to remove the overscroll animation, since
 it is conceptually equivalent to calling the stop method without
 permission to change scene.

Note about the test: we changed the overscroll implementation and the
overscroll test (scrollStartedInScene_doOverscrollAnimation) is still
working.

Test: atest SceneGestureHandlerTest
Bug: 291025415
Change-Id: Id0ef8ab9d7855452a3d6c8f3581064d80f32c14a