Don't use transient states for wallpaper animation.

The WindowManagerService member mLowerWallpaperTarget is not stable
throughout an app transition. Relying on it to be stable causes the
intra-wallpaper animation to start out right but after the windows
have been relayed out there is no longer a lower wallpaper target.
This causes the wallpaper to start tracking the animation of the
current wallpaper target rather than remain stable.

Switching to a new variable that saves the state of wallpaper
animation at the start of the animation fixes bug 11240590.

Change-Id: I336a59c47665fcf61019f567b8663956ff0e4940