js: Get idle monitor from backend

Its ownership has moved from dangeling to owned by the backend, which
has reflected the API is accessible by. Adapt to this change.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1838>

js/ui/messageTray.js

@@ -855,7 +855,7 @@ var MessageTray = GObject.registerClass({
 
         this._userActiveWhileNotificationShown = false;
 
-        this.idleMonitor = Meta.IdleMonitor.get_core();
+        this.idleMonitor = global.backend.get_core_idle_monitor();
 
         this._useLongerNotificationLeftTimeout = false;