Track animating apps up until animation starts.

ActivityStack sets the mNextAppTransition value before moving the apps
to their proper position. This makes the test for mNextAppTransition
being TRANSITION_UNSET incorrect. Better to just look for animating.

Fixes bug 6565418.

Change-Id: I4651b7d3678e46fc712a8e901992fb01b94c415e
1 file changed