messageTray: Use the bottom monitor's fullscreen state for rate limiting

It makes more sense to use the monitor the tray is on, rather than the
primary monitor. This also matches us with whether we can open the tray
from a barrier/dwell or not.

https://bugzilla.gnome.org/show_bug.cgi?id=695659

js/ui/layout.js

@@ -976,8 +976,9 @@ const LayoutManager = new Lang.Class({
                     !metaWindow.appears_focused)
                     metaWindow.minimize();
             }
-            this.emit('primary-fullscreen-changed', this.primaryMonitor.inFullscreen);
         }
+
+        this.emit('fullscreen-changed');
     },
 
     _updateRegions: function() {