Clean up windows which stuck in animatingExit state.

WM delays to destroy window surface if the window is running an exit
animation, and clean it up after the animation finishes. In case the
clean up process was not triggered, we do it before starting an app
transition.

Note that this is a safe guard for potential bugs. The surface shall
be destroyed as soon as an animation finishes, so the clean up should
have been done before an app transition starts.

Bug: 205335975
Test: atest AppTransitionControllerTest
Change-Id: I9c37ab9ebc57ef48827df25ecc52bf09101ad419