Restore flags to disable Fabric background executor

Summary:
We want to fix the order of execution of layout, layout effects and passive effects in React Native, but the use of the Fabric background executor for layout complicates this (and other things).

This brings back a flag to disable this background thread to do layout synchronously in JavaScript, before the execution of layout effects and passive effects.

This is expected to regress performance on some screens, so we need to address the antipatterns in those screens before shipping this.

Changelog: [internal]

Reviewed By: javache

Differential Revision: D39727131

fbshipit-source-id: 4323b089234d3304ca3bfe5697668fb44ac64c12

React/Fabric/RCTSurfacePresenter.mm

@@ -301,7 +301,9 @@ - (RCTScheduler *)_createScheduler
     return std::make_unique<MainRunLoopObserver>(activities, owner);
   };
 
-  toolbox.backgroundExecutor = RCTGetBackgroundExecutor();
+  if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:enable_background_executor_ios")) {
+    toolbox.backgroundExecutor = RCTGetBackgroundExecutor();
+  }
 
   toolbox.synchronousEventBeatFactory =
       [runtimeExecutor, runtimeScheduler = runtimeScheduler](EventBeat::SharedOwnerBox const &ownerBox) {

ReactAndroid/src/main/jni/react/fabric/Binding.cpp

@@ -472,8 +472,11 @@ void Binding::installFabricUIManager(
   toolbox.synchronousEventBeatFactory = synchronousBeatFactory;
   toolbox.asynchronousEventBeatFactory = asynchronousBeatFactory;
 
-  backgroundExecutor_ = JBackgroundExecutor::create("fabric_bg");
-  toolbox.backgroundExecutor = backgroundExecutor_;
+  if (reactNativeConfig_->getBool(
+          "react_fabric:enable_background_executor_android")) {
+    backgroundExecutor_ = JBackgroundExecutor::create("fabric_bg");
+    toolbox.backgroundExecutor = backgroundExecutor_;
+  }
 
   animationDriver_ = std::make_shared<LayoutAnimationDriver>(
       runtimeExecutor, contextContainer, this);

ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -395,8 +395,9 @@ jsi::Value UIManagerBinding::get(
               RuntimeSchedulerBinding::getBinding(runtime);
           auto surfaceId = surfaceIdFromValue(runtime, arguments[0]);
 
-          if (runtimeSchedulerBinding &&
-              runtimeSchedulerBinding->getIsSynchronous()) {
+          if (!uiManager->backgroundExecutor_ ||
+              (runtimeSchedulerBinding &&
+               runtimeSchedulerBinding->getIsSynchronous())) {
             auto weakShadowNodeList =
                 weakShadowNodeListFromValue(runtime, arguments[1]);
             auto shadowNodeList =