Correctly batch reportMount calls (#50090)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50090

Changelog: [internal]

I refactored `FabricUIManager` in D54547194 / https://github.com/facebook/react-native/pull/43337 and accidentally removed setting this flag to avoid scheduling redundant tasks in the UI thread to report mount. This fixes it.

Reviewed By: javache

Differential Revision: D71387374

fbshipit-source-id: cad8a3ead2434738325560902cbab817e5d5dde7

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -1263,6 +1263,8 @@ public class FabricUIManager
       }
 
       if (!mMountNotificationScheduled && !mMountedSurfaceIds.isEmpty()) {
+        mMountNotificationScheduled = true;
+
         // Notify mount when the effects are visible and prevent mount hooks to
         // delay paint.
         UiThreadUtil.getUiThreadHandler()