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/screenShield.js

@@ -138,7 +138,7 @@ var ScreenShield = class {
                                                       fadeFactor: 1 });
         this._shortLightbox.connect('notify::active', this._onShortLightbox.bind(this));
 
-        this.idleMonitor = Meta.IdleMonitor.get_core();
+        this.idleMonitor = global.backend.get_core_idle_monitor();
         this._cursorTracker = Meta.CursorTracker.get_for_display(global.display);
 
         this._syncInhibitor();