Themes: Allow clearing the wallpaper and ls wallpaper

By providing an empty string for the MODS_LAUNCHER and
MODS_LOCKSCREEN components, the theme service will clear out any
existing wallpaper the user had set.  This provides the same
functionality as if the user was picking "No wallpaper" from the
picker.

Change-Id: I0c349ad4afa2bcef2c302105315709786510c0aa