Flutter + Dart. I did minimal Android development with Java before. Never learned Kotlin. Recently learned Flutter. So, it is a biased opinion. But, I feel Flutter is really cool for building cross platform apps.
I became professional android developer in 2010 and wrote android apps for my startup until recently; Since I was comfortable with Java I was using it, just like how there are still iOS developers who write in Objective-C instead of using Swift.
But just as Swift is the future of iOS, Kotlin is the future of Android and anyone starting new with Android development should use that IMO.
I personally, got bored of mobile app development, if I have to develop something in future, I would choose Flutter just to save some time i.e. if Google doesn't kill it by then.
Another update for flutter. It's just so much easier IMO, and only drop down to Java when I really need to.
Learn basics of software enginering (OOP, FP, desig patterns, etc) and then mastering Java or Kotlin is easy.
Anyway, I am doing both of them. If the project is old, it is Java, if a new project then if everyone in the team knows then we pick Kotlin.
But, as other people mentioned in the comments, I am experimentting with Flutter lately and it looks good so far.
I have worked with both Java and Kotlin for App development. I think it is easier to get started with Kotlin but if you have knowledge of concepts of Java such as OOP, JDBC etc. , you'll have an added advantage.
For Android developers, Java has always been the number one choice for their Android app development. But ever since Google announced Kotlin as their official language for Android app development in the Google I/O conference, 2017, there was utter confusion on which to choose: Java or Kotlin?