Fixed retrieving non-initialized wallpaper information

When an attempt is made to retrieve wallpaper information such
as getWidthHint during user switching, the wallpaper may be
uninitialized and therefore throw NullPointerException.

Resolved by initializing wallpaper for current active user
if not yet initialized or returning null if applicable.

Change-Id: I1d3931379cbf4ea82cd25bc813898bbe4fb65a67