Stop preallocation views on the main thread

Summary:
There is no reason to allocate views ahead of time on the main thread.
There is a chance that this view will not be mounted and we are not saving any time because it's a sequential process anyway (because we are doing it on the main thread). Moreover, the switching context can only slowdown JS execution.

Reviewed By: JoshuaGross

Differential Revision: D14026379

fbshipit-source-id: 2dbe93ab32b611fae942468e7812b78afeaf34fc

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -191,6 +191,10 @@ public class FabricUIManager implements UIManager, LifecycleEventListener {
 
   @DoNotStrip
   private void preallocateView(final int rootTag, final String componentName) {
+    if (UiThreadUtil.isOnUiThread()) {
+      // There is no reason to allocate views ahead of time on the main thread.
+      return;
+    }
     synchronized (mPreMountItemsLock) {
       ThemedReactContext context =
         Assertions.assertNotNull(mReactContextForRootTag.get(rootTag));