A Xamarin.Android app with Unity as a lib (a new feature in Unity 2019.3.a2, see this blog post and these steps).
So you can now easily add a Unity scene (e.g. ARCore scene) to any native or Xamarin app via
Unity stuff (
Xamarin stuff (including a few Android Support libs): 8Mb
Total APK size: 24.2Mb (IL Link + proGuard)