Request a wallpaper update pass when wallpaper target is set to visible

Usually wallpaper target gets updated when some wallpaper target window
finishes drawing. However in some cases, Recents app could be set to
visible again before its stopped. (Which could happen when we started
opening transition into some app with a saved surface, but the app draws
so slow so that when user pressed Recents button again, the app still
hasn't delivered first frame.) In this case, the surface is already
drawn and we won't get a finish drawing again. We need to make sure the
wallpaper target is updated.

bug: 27742244
Change-Id: I8ff53f15f95bae8a99a5a0fd11e24e0186dc3345