Fix issue #2170897: wallpaper touch-up event not seen when exiting lock screen

Make sure to deliver events to the wallpaper until the final up.

Also fix behavior in the case where a window goes away while the pointer is still
down in it, which is a fairly novel situation introduced by the new lock screen.

Also add infrastructure for delivering motion events during preview.

Change-Id: I0de7979be27e00caf0b1eff794ea899a815142f6