Background: allow passing the settings schema from outside

This way it is possible to use different settings for different
backgrounds.

https://bugzilla.gnome.org/show_bug.cgi?id=688210
ScreenShield: use the screensaver background

Now that that's configurable in the control center, we should
use the appropriate background here.

https://bugzilla.gnome.org/show_bug.cgi?id=688210