Preventing release animation targets due to unsynchronize.

Users can start another animation by reuse existing animation targets
during post-commit animations while canceling a previous animation.
However, because RemoteAnimationFinishedStub can be called from a
binder call, access to these animation targets must be synchronized.
Secondly, since the animation targets remain the same until the
previous animation finishes, we can disregard any previous
RemoteAnimationFinishedStub calls if the animation controller has
already requested another animation.

Flag: EXEMPT bugfix
Bug: 386794627
Test: re-start back animation animation in post-commit, verify the
animation leashes won't be released before animation finish.

Change-Id: I44f16202f08f2ee4144621c55b1bf0e76ea3f815