SettingsProvider: add default overlay entry for SEND_ACTION_APP_ERROR

Add a default entry for Settings.Global.SEND_ACTION_APP_ERROR so that it
can be overlayed elsewhere if needed.

Change-Id: Ieffc2f4a1ad8ad0df04a9e33b575150a680478af
Signed-off-by: Roman Birg <roman@cyngn.com>
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