How to make a single Android activity in the manifest but use one of two implementations when launched?

My Android app has one main activity today declared in the manifest but the design team wants to build a replacement that is very different in both look and function. We want to perform A-B testing based on a feature flag so some users see the old activity at launch and some see the new activity at launch and we can control this remotely. We already have ability to remotely control the feature flags today already; the value is saved in SharedPreferences.

What is the best way to keep a single activity in the Android manifest and then have it choose one of two different implementations before anything is displayed?

For XML based apps, one of the method that I implemented in one of my app was that I created two fragments and based on the flag, I simply navigated to the required fragment on startup (Jetpack Navigation library can make fragment transactions simple). Since each UI will have its own separate implementation, code will be much cleaner and if the app is based on MVVM architecture, you can also share the view models to share the business logic depending upon your requirements.

Similarly, In Jetpack Compose, it would be a fairly simple to navigate to the composable based on the flag. View Models will work the same as in XML based apps.