js: Get monitor manager from backend

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2557>

js/ui/windowManager.js

@@ -56,7 +56,8 @@ class DisplayChangeDialog extends ModalDialog.ModalDialog {
 
         this._wm = wm;
 
-        this._countDown = Meta.MonitorManager.get_display_configuration_timeout();
+        const monitorManager = global.backend.get_monitor_manager();
+        this._countDown = monitorManager.get_display_configuration_timeout();
 
         // Translators: This string should be shorter than 30 characters
         let title = _('Keep these display settings?');