Change wallpaper format to OPAQUE

The wallpaper format in this case is RGBX_8888 (00000002). It's fairly common that it becomes this,
but depends on how you've used the system and which wallpapers you've set.

It could be 00000005 already on your device.
RGBX_8888 is not handled by DSS, so we tell the system to just request an opaque surface by default.
The GPU driver will return a 00000005 surface that the system likes better.
This change increases the chance that DSS can handle the wallpaper, and if not,
makes it more blitter friendly.

Credits to jonasl. See http://forum.xda-developers.com/showpost.php?p=45088738&postcount=585