Calls cancel on mDimAnimator to ensure that ref counts are accurate.

When we used cancelAnimationWithoutCallbacks on mDimAnimator,
the decrementOnAnimationEnd listener is removed. This created
a bug where the ReferenceCountedTrigger was not able to
flush its last decrement runnables.

ie. The Close icon was not always visible for the apps in Recents/Overview.

Bug: 37132803
Test: Manual testing.
Change-Id: Idbb1a1e42313db71328990577b3163b0d16b28d6