Introduce TransitionState.Transition.finish() (1/2)

This CL adds a finish() method to Transition that can be used to force a
transition to finish/settle and wait for that transition to end.

This new API should be generally useful and will be used to better
support interruptions.

Bug: 290930950
Test: atest DraggableHandlerTest
Test: atest SceneTransitionLayoutStateTest
Flag: N/A
Change-Id: Ib637485469d617d696ee56da7f1066b4283ecdfe