Schedule React revision merge to happen before dispatching events

Summary:
Changelog: [ANDROID][FIXED] Schedule React revision merge to happen on `DISPATCH_UI` choreographer queue, before dispatching events

Before the introduction of branching, it was possible to handle synchronous events on the same frame they were dispatched. Branching broke that in two ways:
1. Merge was always scheduled on the end of the event loop (addressed by the previous diff)
2. Merge was scheduled using `runOnUIThread`, which turned out to be after the `DISPATCH_UI` choreographer phase, thus after the dispatch of events.

Differential Revision: D100966623

Author: j-piasecki

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

@@ -101,6 +101,7 @@
 import java.util.Map;
 import java.util.Queue;
 import java.util.Set;
+import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
@@ -192,6 +193,13 @@ public class FabricUIManager
   @ThreadConfined(UI)
   private final Set<SynchronousEvent> mSynchronousEvents = new HashSet<>();
 
+  /**
+   * Queue of surface IDs that need their React revision merged. Drained during doFrame so that
+   * synchronous events dispatched by the merge are processed in the same frame.
+   */
+  private final ConcurrentLinkedQueue<Integer> mPendingReactRevisionMerges =
+      new ConcurrentLinkedQueue<>();
+
   /**
    * This is used to keep track of whether or not the FabricUIManager has been destroyed. Once the
    * Catalyst instance is being destroyed, we should cease all operation here.
@@ -956,13 +964,7 @@ private void scheduleReactRevisionMerge(int surfaceId) {
         mBinding.mergeReactRevision(surfaceId);
       }
     } else {
-      UiThreadUtil.runOnUiThread(
-          () -> {
-            FabricUIManagerBinding binding = mBinding;
-            if (binding != null) {
-              binding.mergeReactRevision(surfaceId);
-            }
-          });
+      mPendingReactRevisionMerges.add(surfaceId);
     }
   }
 
@@ -1558,6 +1560,18 @@ public void doFrameGuarded(long frameTimeNanos) {
         return;
       }
 
+      // Drain pending React revision merges first so that animations,
+      // preallocation, and mount items operate against the latest revision.
+      {
+        FabricUIManagerBinding binding = mBinding;
+        if (binding != null) {
+          Integer mergeSurfaceId;
+          while ((mergeSurfaceId = mPendingReactRevisionMerges.poll()) != null) {
+            binding.mergeReactRevision(mergeSurfaceId);
+          }
+        }
+      }
+
       // Drive any animations from C++.
       // There is a race condition here between getting/setting
       // `mDriveCxxAnimations` which shouldn't matter; it's safe to call