screenShield: Do not lock the screen when locked down

When using the 'disable-lock-screen' setting to lock down the screen
lock, the expectation is that users cannot lock the screen. However
as it turns out, all the setting currently does is hiding the lock
button in the system menu and making the lock settings in the privacy
panel inactive. That means that if the 'lock-screen-enabled' setting
isn't disabled and locked down as well, we will just continue to
lock the screen on inactivity - not to mention the keyboard shortcut
that isn't subject to that setting anyway.

Instead of expecting administrators to hunt down every possible way
of locking the screen and disabling it individually, we can easily
handle all cases by refusing to lock the screen when disabled by the
lockdown settings.

https://bugzilla.gnome.org/show_bug.cgi?id=780212