Add DefaultReactHost for Template (#39585)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39585

So that to enable Bridgeless in Template android we only need to override the ReactHostInterface variable in Template's application, see D49464580

Changelog:
[Android][Changed] - Add DefaultReactHost

Reviewed By: cortinico

Differential Revision: D49463901

fbshipit-source-id: ff6ef1bef8626996a3658597d1519eec310c77c3

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultReactHost.kt

@@ -0,0 +1,59 @@
+/*
+ * 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 android.content.Context
+import com.facebook.react.ReactPackage
+import com.facebook.react.bridge.JSBundleLoader
+import com.facebook.react.common.annotations.UnstableReactNativeAPI
+import com.facebook.react.fabric.ComponentFactory
+import com.facebook.react.interfaces.ReactHost
+import com.facebook.react.interfaces.exceptionmanager.ReactJsExceptionHandler
+import com.facebook.react.runtime.JSCInstance
+import com.facebook.react.runtime.ReactHostImpl
+import com.facebook.react.runtime.hermes.HermesInstance
+
+@UnstableReactNativeAPI
+object DefaultReactHost {
+  private var reactHost: ReactHost? = null
+
+  @JvmStatic
+  fun getDefaultReactHost(
+      context: Context,
+      packageList: List<ReactPackage>,
+      jsMainModulePath: String = "index",
+      jsBundleAssetPath: String = "index",
+      isHermesEnabled: Boolean = true
+  ): ReactHost {
+    if (reactHost == null) {
+      val jsBundleLoader =
+          JSBundleLoader.createAssetLoader(context, "assets://$jsBundleAssetPath", true)
+      val jsEngineInstance = if (isHermesEnabled) HermesInstance() else JSCInstance()
+      val defaultReactHostDelegate =
+          DefaultReactHostDelegate(
+              jsMainModulePath = jsMainModulePath,
+              jsBundleLoader = jsBundleLoader,
+              reactPackages = packageList,
+              jsEngineInstance = jsEngineInstance,
+              turboModuleManagerDelegateBuilder = DefaultTurboModuleManagerDelegate.Builder())
+      val reactJsExceptionHandler = ReactJsExceptionHandler { _ -> }
+      val componentFactory = ComponentFactory()
+      DefaultComponentsRegistry.register(componentFactory)
+      // TODO: T164788699 find alternative of accessing ReactHostImpl for initialising reactHost
+      reactHost =
+          ReactHostImpl(
+              context,
+              defaultReactHostDelegate,
+              componentFactory,
+              true,
+              reactJsExceptionHandler,
+              true)
+    }
+    return reactHost as ReactHost
+  }
+}