ScreenShield: don't reset the lock screen animation if it's already happening

ScreenShield: don't reset the lock screen animation if it's already happening

When you click Suspend from the user menu, the following things happen:
- we lock the screen internally by calling Main.screenShield.lock() and waiting
  for lock-screen-shown
- logind emits a Lock signal, which causes us to lock again
- gnome-settings-daemon notices PrepareForSleep, and calls org.gnome.ScreenSaver.Lock,
  just in case, so we lock once more
This means that, if you're lucky, you can see the curtain fall down multiple times,
as each .lock() call resets the animation.

https://bugzilla.gnome.org/show_bug.cgi?id=690858