Add awaitFling lambda to Controller.onStop()

This CL adds a way for NestedDraggable.Controller.onStop() to wait for
the overscroll (wrapping the call to onStop()) to be finished. This will
be used to ensure that a STL transition is marked as finished when its
overscroll effect is finished.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT NestedDraggable is not used yet
Change-Id: I5ec36d85187561ebc663a3aba77649e41dfaf031