mpris: Disallow media section on login screen

The 'gdm' user is not going to run a media player, so there is no
point in allowing the corresponding section on the login screen.

All other sections are already disabled, so this is the only reason why
we end up with the message list instead of only showing the calendar.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2241

js/ui/mpris.js

@@ -251,6 +251,10 @@ class MediaSection extends MessageList.MessageListSection {
         return !this.empty && Calendar.isToday(this._date);
     }
 
+    get allowed() {
+        return !Main.sessionMode.isGreeter;
+    }
+
     _addPlayer(busName) {
         if (this._players.get(busName))
             return;