[on_and_off]

Well, Scaloid is a thing, so you might want to check it out. AFAIK, it is not really popular for Android development, but it makes some people really happy, like predictive keyboards creators.

As for Xamarin, it really depends on what you want to do. If this is just a hobby project, well go for it. Otherwise, the only use case where it is really interesting is when you need to write an app for both iOS & Android on the cheap.