Use the standard apps tree for gnome-control-center panels desktop files

gnome-control-center is planning on removing its own tree in the
future. Since it already installs these applications into
/usr/share/applications, just use this for now.

https://bugzilla.gnome.org/show_bug.cgi?id=692483

js/ui/userMenu.js

@@ -822,7 +822,7 @@ const UserMenuButton = new Lang.Class({
 
     _onMyAccountActivate: function() {
         Main.overview.hide();
-        let app = Shell.AppSystem.get_default().lookup_setting('gnome-user-accounts-panel.desktop');
+        let app = Shell.AppSystem.get_default().lookup_app('gnome-user-accounts-panel.desktop');
         app.activate();
     },