Add DefaultReactHost for Template (facebook#39585)

Summary:

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

Differential Revision: D49463901

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

@@ -0,0 +1,60 @@
+/*
+ * 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
+class DefaultReactHost {
+  companion object {
+    private var reactHost: ReactHost? = null
+
+    fun getDefaultReactHost(
+        context: Context,
+        jsMainModulePath: String,
+        jsBundleAssetPath: String = "",
+        packageList: List<ReactPackage>,
+        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
+    }
+  }
+}