Restore wallpaper positioning between apps.

Simplification where wallpaper was behind all apps didn't work when
keyguard and associated wallpaper needed to be above phone screen when
phone screen animated in and out. Instead phone screen was instantly
hiding the wallpaper.

Fixes most of bug 10932680.

This fixes the wallpaper disappearing as soon as the animation begins
when going from keyguard to phone. There remains jank going from phone
to lockscreen where the animation is not occurring and the phone
blanks out immediately.

Change-Id: Ie5f464acb2f6cefd2fb91f3b920a687ec7c15d76