Add wallpaper input consumer to WindowManagerService

This is an input consumer similar to the one used when hiding the navbar,
but placed above wallpapers. It might be useful for processing touch
events over "desktop" in freeform MW mode.

Re-landing I9d6d28a624f750ad48fc39f9b149dd1f989cceba after fixing build.

Bug:26688904
Change-Id: I89fdabd9c72cdd4a1d7ca626c33ddc99ddea97f9