I think you should ask yourself. If you have experience with core Java, go with that. If you have experience in web development or React/React-Native, then go with that.
You could learn Kotlin on the fly too! If you are a beginner, go with Java/Kotlin. There is actually a tutorial a for Kotlin in the developer guides. This path will be easier and is more supported. FYI, even React native internally uses Android SDK to display your HTML and execute your JS. So, I would recommend learning Android via Java. That will also help you understand how Android works internally.