I don't know what they are using, but you could take a look at this:Please, does anyone know what method or way they are using? Please provide a link, I would much rather use Swift than Java. (and its not 1. april)
http://elementscompiler.com/elements/silver/
It allows you to write Swift code targeting Android, plain old Java (i.e. you can write Java Swing apps in Swift if you really wanted to), and the .NET platforms.
I've done a little playing with Silver and their Fire IDE. Lots of room for improvement, but it *is* possible to write Swift to target Android (it compiles to the JVM). Note that there are some limitations with their Swift implementation due to how the JVM works, but they are not insurmountable.
The big thing is that they don't try to provide you with a cross-platform way to write everything including the UI. You'll still have to write separate UI code for each platform, but you should be able to reuse most (all) of your model layer code.