udev: Create udev in MetaBackend

So we can use it for backlight control in both the native and X11
backends.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4130>

src/tests/native-kms-headless-start.c

@@ -72,7 +72,7 @@ static void
 meta_test_headless_monitor_connect (void)
 {
   MetaBackend *backend = meta_context_get_backend (test_context);
-  MetaUdev *udev = meta_backend_native_get_udev (META_BACKEND_NATIVE (backend));
+  MetaUdev *udev = meta_backend_get_udev (backend);
   MetaMonitorManager *monitor_manager =
     meta_backend_get_monitor_manager (backend);
   g_autolist (GObject) udev_devices = NULL;