ScreenShield: hide the lightboxes when resuming from suspend

We show a lightbox when we suspend, to animate the fading to black
caused by turning off the monitors, but we need to hide it when
coming back, otherwise the user is just staring at a black screen
it until he moves the mouse or presses a key.

https://bugzilla.gnome.org/show_bug.cgi?id=706654

js/ui/screenShield.js

@@ -732,6 +732,8 @@ const ScreenShield = new Lang.Class({
             this.lock(true);
         } else {
             this._inhibitSuspend();
+
+            this._onUserBecameActive();
         }
     },