ScreenShield: honor lock-delay GSettings key

org.gnome.desktop.screensaver.lock-delay contains the grace period
of the screensaver: if deactivated within that many seconds from the
start of the idle period, the shell should not prompt for a password.
This setting correspond to the "Lock screen after" combo in screen
and privacy panels.

https://bugzilla.gnome.org/show_bug.cgi?id=690766