Snackbars show lightweight feedback to users and are easy to set up. This tutorial shows a basic case, action handling, and UI customization for Android.
RubyFlux statically compiles Ruby into Java. It has awesome performance and runs everywhere Java does with no runtime dependencies, but it isn't done yet.
The platform documentation for building an Android APK is sparse. Here's a complete example of how to build an APK for yourself if you should require it.