put PR#41194 behind gating (facebook#41469)

Summary:

Changelog: [Internal]

PR#41194 (facebook#41194) introduces a new callsite to `RCTUnsafeExecuteOnMainQueueSync` in the module init path, which increases risk for deadlock. let's gate it

Differential Revision: D51274859

Author: philIip

packages/react-native/React/Modules/RCTUIManager.h

@@ -20,6 +20,8 @@
  */
 RCT_EXTERN NSString *const RCTUIManagerWillUpdateViewsDueToContentSizeMultiplierChangeNotification;
 
+RCT_EXTERN void RCTUIManagerSetDispatchAccessibilityManagerInitOntoMain(BOOL enabled);
+
 @class RCTLayoutAnimationGroup;
 @class RCTUIManagerObserverCoordinator;
 

packages/react-native/React/Modules/RCTUIManager.m

@@ -61,6 +61,12 @@ static void RCTTraverseViewNodes(id<RCTComponent> view, void (^block)(id<RCTComp
 NSString *const RCTUIManagerWillUpdateViewsDueToContentSizeMultiplierChangeNotification =
     @"RCTUIManagerWillUpdateViewsDueToContentSizeMultiplierChangeNotification";
 
+BOOL kDispatchAccessibilityManagerInitOntoMain = NO;
+void RCTUIManagerSetDispatchAccessibilityManagerInitOntoMain(BOOL enabled)
+{
+  kDispatchAccessibilityManagerInitOntoMain = enabled;
+}
+
 @implementation RCTUIManager {
   // Root views are only mutated on the shadow queue
   NSMutableSet<NSNumber *> *_rootViewTags;
@@ -181,7 +187,10 @@ - (void)setBridge:(RCTBridge *)bridge
   }
 
   // This dispatch_async avoids a deadlock while configuring native modules
-  dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0), ^{
+  dispatch_queue_t accessibilityManagerInitQueue = kDispatchAccessibilityManagerInitOntoMain
+      ? dispatch_get_main_queue()
+      : dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0);
+  dispatch_async(accessibilityManagerInitQueue, ^{
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(didReceiveNewContentSizeMultiplier)
                                                  name:@"RCTAccessibilityManagerDidUpdateMultiplierNotification"