Avoid unnecessarily redrawing the wallpaper.

Added some logic to ImageWallpaper to avoid redrawing the wallpaper
when it is not necessary.  Even with the resize fixes, we were still
redrawing the wallpaper an extra 5 times due to visibility and
offset changes which had no real effect.

Change-Id: Ifa39b7bb8c0f7a6a41bc81e00cc2159440f529c5