Why would I ever care about Go for Android when I can write apps in Kotlin? Kotlin is fully interoperable with Java. I don't have much experience with Swift but Kotlin is so nice. It has the best features from numerous languages such as Ruby. C#, etc. It's being made by Jetbrains who makes the the core of Android Studio so it will have support. Kotlin is also nearly as fast at runtime as Java with a tiny 200kb runtime. Almost all the magic happens at build time so your build time will be a little longer. Kotlin also works with any existing Java library, even annotation processing libs.
Again, why would any Android developer choose Swift? Kotlin also runs fine on the server and it even compiles to Javascript.
If I were a startup doing an iOS, Android, and a backend I would do either Go or Rust on the backend. You will be able to distribute a native lib with your Android or iOS that shares network and model logic. Go and Rust both are planning on supporting cross compilation to iOS and Android.
That would leave you with Kotlin/Groovy/Clojure/Scala for the view layer of your Android app if you choose not use Java. Which is a good idea since there is no indication from Google Java 8 will be supported which means no lambdas unless you use an alt JVM language. On iOS you write your view layer in Swift. Both apps use the shared binary library.