Wait for visible wallpaper drawn before starting app transitions

If the opening app has wallpaper, when the closing app starts hiding,
the wallpaper would be revealed. It would be nice if we play the app
transitions while opening apps, closing apps, and visible wallpapers
are all drawn.

https://code.google.com/p/android/issues/detail?id=150811

Change-Id: I3c7d140f6f6e43e18119e48f9cab441ee96b17e5