Bug 1015853 - Changing theme should update wallpaper image.

Fetches the image defined as homescreen image in a theme apps
wallpaper.json file and sets it as the current wallpaper image.
As the function is responsible for getting the path of the wallpaper, it hosuld not call to `loadWalpaper` directly. I would suggest to return a promise that wraps the path of the wallpaper.