Put animation background behind visible wallpaper

The new wallpaper positioning associated with multiple activity stacks
put animation backgrounds in front of the wallpaper. This caused
hideous jank. Testing for visible wallpaper and moving the background
behind it fixes the jank.

Fixes bug 10078282.
Possibly fixes bug 10247094.

Change-Id: I3f4e07accd7276d59725192081904b791f77781f