Cancel Pip animation if stack is already removed

It is possible for the pinned stack to be removed before the system has
a chance to run the scheduled bounds animation on it. In this case we
should just cancel the animation.
Also fixed:
- Issue where we are synchronizing on the wrong object in
PinnedActivityStack.updatePictureInPictureModeForPinnedStackAnimation
- Issue where the enterPipRunnable in
ActivityTaskManagerService.enterPictureInPictureMode wasn't synchronized

Change-Id: I9717ac4b5849eafa6332fb0043310a0e8387a05f
Fixes: 110255768
Test: atest ActivityManagerActivityVisibilityTests#testHomeVisibleOnActivityTaskPinned