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/magnifier.js

@@ -750,7 +750,7 @@ var ZoomRegion = class ZoomRegion {
         this._xCaret = 0;
         this._yCaret = 0;
 
-        this._pointerIdleMonitor = Meta.IdleMonitor.get_core();
+        this._pointerIdleMonitor = global.backend.get_core_idle_monitor();
         this._scrollContentsTimerId = 0;
     }