Remove @UiThread annotation from MountingManager.addRootView()

Summary: Removing the UiThread annotation from MountingManager.addRootView() because it doesn't seem like it's needed.

Reviewed By: mdvacca

Differential Revision: D15945819

fbshipit-source-id: 0cb9bfb2fe2889001f1c061a07518cf9734f0c16

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountingManager.java

@@ -51,7 +51,6 @@ public class MountingManager {
     mViewFactory = new ViewManagerFactory(viewManagerRegistry);
   }
 
-  @UiThread
   public void addRootView(int reactRootTag, View rootView) {
     if (rootView.getId() != View.NO_ID) {
       throw new IllegalViewOperationException(