Reset mActiveController to null in DragLayer.onInterceptTouchEvent().

Otherwise the previous active controller will continue to handle touch
events even if it doesn't re-intercept future touches. For instance,
All Apps was handling the swipe gesture after DragLayer intercepted to
close a shortcuts container, which led to the weird behavior described
in the bug.

Bug: 30590854
Change-Id: I247b39b03d336a04659f6ce644380bf3cef8de3f