Simplify new app template for bridgeless (#40929)

Summary:
Pull Request resolved: facebook/react-native#40929

This diff reduces the footprint that bridgeless is imposing on the new app template. Specifically:
- I've created a `.toReactHost` method that converts a DefaultReactNativeHost to a DefaultReactHost
- I've updated RN Tester to use the same setup as the New App template which reduces code duplication.

I also had to remove a couple of `UnstableReactNativeAPI` as those were bleeding in the new app template.
I don't think we should ask users to opt-in in `UnstableReactNativeAPI` in the New App template itself as
this means that all the apps will get this opt-in.
Instead we should keep it only for specific APIs that we want the users to opt into.

Changelog:
[Internal] [Changed] - Simplify new app template for bridgeless

Reviewed By: cipolleschi, luluwu2032

Differential Revision: D50227693

fbshipit-source-id: e86c54d5156cc27f1f898b43ca89c57d5cf148b8

Original-Commit: facebook/react-native@a7f9080

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

@@ -6,13 +6,11 @@ import com.facebook.react.ReactApplication
 import com.facebook.react.ReactHost
 import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackage
-import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.defaults.DefaultNewArchitectureEntryPoint.load
-import com.facebook.react.defaults.DefaultReactHost
+import com.facebook.react.defaults.DefaultReactHost.getDefaultReactHost
 import com.facebook.react.defaults.DefaultReactNativeHost
 import com.facebook.soloader.SoLoader
 
-@UnstableReactNativeAPI
 class MainApplication : Application(), ReactApplication {
 
  override val reactNativeHost: ReactNativeHost =
@@ -32,12 +30,7 @@ class MainApplication : Application(), ReactApplication {
  }
 
  override val reactHost: ReactHost
- get() =
- DefaultReactHost.getDefaultReactHost(
- context = this,
- packageList = PackageList(this).packages,
- jsMainModulePath = "index",
- isHermesEnabled = BuildConfig.IS_HERMES_ENABLED)
+ get() = getDefaultReactHost(this.applicationContext, reactNativeHost)
 
  override fun onCreate() {
  super.onCreate()