Fix #2313382: SECURITY: Live wallpapers get touch events through the lock screen.

This is a quick and dirty hack to not deliver touch events to the wallpaper when
they are being sent to the keyguard.  Perhaps we should have a separate window
flag for this, but...  bleah.  Maybe later.  Or maybe I'll use that secure flag.
Or something.

Change-Id: Ifd95b9f5b10db24a0854a93b925a833b24331b4c