WallpaperManagerService: use a single file observer

In order to clean up logic, 2a80ec9a split up to have two separate file
observers, since there are two separate objects, one for kg, one for
regular wallpaper. However, there can only be one file observer
registered for a directory. This was causing the keyguard observer
getting all the inotify events since it was the last one registered,
which stopped regular wallpaper change detections until a reboot.

Move the file observer logic out of the wallpaper info and make it
work for both.

Change-Id: I8fd4976fb4731abe52dfcc8064af2edc42b274f9
Signed-off-by: Roman Birg <roman@cyngn.com>