messageTray: Only update keyboardVisible as necessary

This fixes a case of _updateState() being called recursively,
resulting in stray grab()/ungrab() calls the leave the entire
desktop in a stuck focus state.

https://bugzilla.gnome.org/show_bug.cgi?id=683546

js/ui/messageTray.js

@@ -1868,6 +1868,9 @@ const MessageTray = new Lang.Class({
     },
 
     _onKeyboardVisibleChanged: function(layoutManager, keyboardVisible) {
+        if (this._keyboardVisible == keyboardVisible)
+            return;
+
         this._keyboardVisible = keyboardVisible;
 
         if (keyboardVisible)