ScreenShield: don't bump the lock screen while animating

Bumping overrides any animation, and causes the installed onComplete
not to run, which in turn breaks suspending from the user menu (as
it listens to lock-screen-shown).
Add a state variable (using an enumeration shared with messageTray)
to control the lock screen, and only bump when the lock is still.

https://bugzilla.gnome.org/show_bug.cgi?id=682746