I've poked around with Kotlin and considered using it for a game project with LibGDX. But, Java is good enough, and if I use K I have to add libraries. I just didn't want to deal with the additional hassle ... not enough reward for the work. Maybe that will change in the future.
Also, Android Studio kind of sucks ! Some real head scratchers like really poor IMO syntax error notifications. Just seems like nobody is doing usability improvements on old components.