services: don't clear wallpaper when SystemUI disconnects.

When we restart SystemUI (or the user), sometimes
the wallpaper would get cleared. This patch just
captures its originating component and
if that is our SystemUI component, ignore the event.

Change-Id: I3e159dc2c013e7d7e552e3c0cef55e3ab970ca87
Signed-off-by: Roman Birg <romanbirg@gmail.com>