Nekromancer

I'm going to assume that you mean if you should use Flutter or Native Kotlin stack for android development. The answer is simple - if all you want to do is android development - go with Kotlin. If you want to go crossplatform - go flutter

Ruby is not in a spectrum of "highly efficient" languages. It's interpreted, pretty slow and not concurrent. Java is very good for like 99% of cases, and it's much faster to work with than Rust. But, if you really want to be "close to the metal", Rust is your language of choice. It does not have garbage collector. Memory safety is guaranteed by the language itself. This is great, but the cost of that is pretty steep learning curve. But it's still a lot of fun when you get a grip of it.

First of all - Android Studio and Visual Studio are IDE's. Tools to create code. What you are asking is programming framework. I assume that when you are talking about Android Studio you mean Native Android Development and by Visual Studio you mean Xamarin.

If you want to create crossplatform app then Native Android Development is NOT a way to go. Xamarin might work for you, BUT - you'd rather recommend you to go with Flutter. It's much more performant than Xamarin, programming model is friendlier for developer and technology seems just more refined. It's also officially supported by google, so no worries about support.