HelloWorld should not use ReactNativeHost (#53399)

Summary:
Pull Request resolved: #53399

We currently create application template that are still using `ReactNativeHost`.
As this is a legacy arch class, we should remove it from the template ASAP so that
users can organically migrate away from it.

I will also update https://github.com/react-native-community/template with
the same changes I'm applying here.

Changelog:
[Internal] [Changed] -

Reviewed By: mdvacca

Differential Revision: D80708461

fbshipit-source-id: e0d4a1f817e6fdddd93405decf77fd115956ec51

Author: cortinico

private/helloworld/android/app/src/main/java/com/helloworld/MainApplication.kt

@@ -12,29 +12,22 @@ import com.facebook.react.PackageList
 import com.facebook.react.ReactApplication
 import com.facebook.react.ReactHost
 import com.facebook.react.ReactNativeApplicationEntryPoint.loadReactNative
-import com.facebook.react.ReactNativeHost
-import com.facebook.react.ReactPackage
 import com.facebook.react.defaults.DefaultReactHost.getDefaultReactHost
-import com.facebook.react.defaults.DefaultReactNativeHost
 
 class MainApplication : Application(), ReactApplication {
 
-  override val reactNativeHost: ReactNativeHost =
-      object : DefaultReactNativeHost(this) {
-        override fun getPackages(): List<ReactPackage> =
-            PackageList(this).packages.apply {
-              // Packages that cannot be autolinked yet can be added manually here, for example:
-              // add(MyReactNativePackage())
-            }
-
-        override fun getJSMainModuleName(): String = "index"
-
-        override fun getUseDeveloperSupport(): Boolean = BuildConfig.DEBUG
+  override val reactHost: ReactHost by
+      lazy(LazyThreadSafetyMode.NONE) {
+        getDefaultReactHost(
+            context = applicationContext,
+            packageList =
+                PackageList(this).packages.apply {
+                  // Packages that cannot be autolinked yet can be added manually here, for example:
+                  // add(MyReactNativePackage())
+                },
+        )
       }
 
-  override val reactHost: ReactHost
-    get() = getDefaultReactHost(applicationContext, reactNativeHost)
-
   override fun onCreate() {
     super.onCreate()
     loadReactNative(this)