Bullet-proofing the drag window initial animation.

It’s unlikely, but still possible that something interesting will happen
during the 150 ms that DragWindows takes to slightly increase in size
before it gets hidden and replaced by the framework DND drag shadow.

This fix covers 2 possible events that can happen during that time:
1. The drag gets cancelled (for example because of an app uninstall).
Here, we eliminate a crash.
2. The user starts dragging the shadow. Before, this would start
scrolling the home page. Now, SystemDragDriver starts processing touch
events exactly as the InternalDragDriver would, until the growing
finishes. Afterwards, it switches to framework dragging.

Bug: 22609426
Change-Id: I73cc09fea45fcdcc6b75b13be1347da3e7c95509