Animate app exit for cases where the app is already hidden before transition

It is possible for an app token to be marked as hidden and exiting
because the previous activity destory is completed before we are
good-to-go with app transition since we are still waiting for the
opening app to be done drawing. In this case we don't apply the
exiting animation since the token is not changing visibility state
during the app transition (it is already hidden and we are telling
it to hide...).
We now allow the animation to load if the token is already hidden
and marked as exiting.

Bug: 22381703
Change-Id: I5c27b6c3c1f810f05b2d716122055c6f12d0a167