screenShield: explicitly load gnome-screensaver in fallback mode.

When running gnome-shell from lightDM, gnome-screensaver is no
longer auto-loaded. As a result the dbus calls for Lock user etc
will fail.

https://bugzilla.gnome.org/show_bug.cgi?id=683060
Bug-Ubuntu: https://launchpad.net/bugs/1064354