Fixed issue with drag resize state when animating pinned stack.

When animating the pinned stack, we set drag resizing on the top
most task in the stack. This has a couple of issues.
- Only the top most task is put in drag sizing mode and all other
task in the stack will not be in resizing mode.
- The top most task of the stack can change during the animation,
so we fail to clear the drag resize flag on the previous top task.

We now track drag sizing at the stack level and have the stack
drag resizing state affect its tasks drag resizing states.

Also added concept of continuing a bounds animation if the same
target called BoundsAnimationController.animateBounds before the
current animation is done. We don't send onAnimationEnd() for the
current animation that is been cancelled and don't send
onAnimationStarted() for the animation that will be replacing it.

Bug: 25672053
Change-Id: I64e89ed09d81e4802dacebc5818dfa1deb0d588f