Fix 5907223: Fix lock screen glitch caused on devices with a system bar

This fixes a regression caused by making the lock screen window slippery where
dragging the target over the system bar would incorrectly cause it to
snap to an invisible target.  It now interprets MotionEvent.ACTION_CANCEL
the same as an "up" event and resets to the initial state.

Change-Id: I9a3c195371d64e1a4613f6f1fb0a043e9a47a601