CM11 Themes: load wp from apk if uri is null

When setting a default theme on first start, the provider may
not have processed the theme and the uri for the wp will be null.
If this happens try to load the wallpaper from the themes assets.

Change-Id: I883b9dd796920e66e4729e78d41cd8899916fa42
(cherry picked from commit fd55c3e3)