messageTray: Use a regular tween when expanding the notification

When a notification becomes expanded, it's either already shown,
or in the process of being shown. Don't set the state to SHOWING
again, which confuses our state machine.

js/ui/messageTray.js

@@ -2755,7 +2755,7 @@ const MessageTray = new Lang.Class({
         } else if (this._notification.y != expandedY) {
             // Tween also opacity here, to override a possible tween that's
             // currently hiding the notification.
-            this._tween(this._notificationWidget, '_notificationState', State.SHOWN,
+            Tweener.addTween(this._notificationWidget,
                              { y: expandedY,
                                opacity: 255,
                                time: ANIMATION_TIME,