Fix missing HUN disappear animation

Make the StackStateAnimator.startAnimationForEvents check if the
AnimationEvents have produced any custom animations, before its
executing the onFinish runnables.

We need this, because since ag/20789485 the ExpandableNotificationRow
starts its HUN appear and disappear animations async.

Fixes: 289495907
Test: Post a HUNning notification, cancel it with clicking its action,
observe the animation

Change-Id: I7146c9bcc6c17a1650675886b83ac27549a09979