Wallpaper: Create new WallpaperObserver on user switch

When switching users we need to create a new WallpaperObserver to
observe changes to the wallpaper and keyguard for the new user,
otherwise the observer will be watching the wrong directory.

Change-Id: Ibb078d7688ba7c6d1519c12f358bb8a2c34b8bf7
TICKET: CYNGNOS-1004
(cherry picked from commit aa7e8ebe)