Introduce the DefaultMainActivityDelegate to simplify enabling/disabling Fabric for new apps. (#34446)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/34446

I'm adding another class to the .defaults package. This will take care of setting the RootView
with Fabric enabled/disabled as well as controlling concurrent root.

Changelog:
[Android] [Added] - Introduce the DefaultMainActivityDelegate to simplify enabling/disabling Fabric for new apps.

Reviewed By: cipolleschi

Differential Revision: D38823181

fbshipit-source-id: 2293b9df6b0d8fa79695bd52a8e0bb46b44c43c8

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultReactActivityDelegate.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.defaults
+
+import com.facebook.react.ReactActivity
+import com.facebook.react.ReactActivityDelegate
+import com.facebook.react.ReactRootView
+
+/**
+ * A utility class that allows you to simplify the setup of a [ReactActivityDelegate] for new apps
+ * in Open Source.
+ *
+ * Specifically, with this class you can simply control if Fabric and Concurrent Root are enabled
+ * for an Activity using the boolean flags in the constructor.
+ *
+ * @param fabricEnabled Whether Fabric should be enabled for the RootView of this Activity.
+ * @param concurrentRootEnabled Whether ConcurrentRoot (aka React 18) should be enabled for the
+ * RootView of this Activity.
+ */
+open class DefaultReactActivityDelegate(
+    activity: ReactActivity,
+    mainComponentName: String,
+    val fabricEnabled: Boolean = false,
+    val concurrentRootEnabled: Boolean = false
+) : ReactActivityDelegate(activity, mainComponentName) {
+
+  /**
+   * Override this method to enable Concurrent Root on the surface for this Activity. See:
+   * https://reactjs.org/blog/2022/03/29/react-v18.html
+   *
+   * This requires to be rendering on Fabric (i.e. on the New Architecture).
+   *
+   * @return Whether you want to enable Concurrent Root for this surface or not.
+   */
+  override fun isConcurrentRootEnabled(): Boolean {
+    return concurrentRootEnabled
+  }
+
+  override fun createRootView(): ReactRootView =
+      ReactRootView(getContext()).apply { setIsFabric(fabricEnabled) }
+}

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultReactNativeHost.kt

@@ -12,6 +12,14 @@ import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackageTurboModuleManagerDelegate
 import com.facebook.react.bridge.JSIModulePackage
 
+/**
+ * A utility class that allows you to simplify the setup of a [ReactNativeHost] for new apps in Open
+ * Source.
+ *
+ * Specifically, for apps that are using the New Architecture, this Default class takes care of
+ * providing the default TurboModuleManagerDelegateBuilder and the default JSIModulePackage,
+ * provided the name of the dynamic library to load.
+ */
 abstract class DefaultReactNativeHost protected constructor(application: Application) :
     ReactNativeHost(application) {
 
@@ -32,7 +40,7 @@ abstract class DefaultReactNativeHost protected constructor(application: Applica
 
   /**
    * Returns the name of the dynamic library used by app on the New Architecture. This is generally
-   * "<applicationname>_appmodules"
+   * "<applicationname>_appmodules" or just "appmodules"
    *
    * If null, we will assume you're not using the New Architecture and will not attempt to load any
    * dynamic library at runtime.