RCTSurface: Use async dispatch to register root shadow view

Summary: We don't really need sync dispatch here. We only need sequential execution of our UIManager blocks.

Reviewed By: rsnara

Differential Revision: D6367069

fbshipit-source-id: cc675aafd6c762506048bcf65c24e54080b013a5

React/Base/Surface/RCTSurface.mm

@@ -311,7 +311,7 @@
   }
 
   RCTUIManager *uiManager = batchedBridge.uiManager;
-  RCTUnsafeExecuteOnUIManagerQueueSync(^{
+  RCTExecuteOnUIManagerQueue(^{
     [uiManager registerRootViewTag:self->_rootViewTag];
 
     RCTSurfaceRootShadowView *rootShadowView =