Android: Convert Java code to Kotlin
The kdeconnect-android codebase is currently a mix of Java and Kotlin. Let's convert what we can to Kotlin.
This should help in a number of places, not least with making the code simpler (hopefully) and reducing the chance of null-based errors. It'll be hard to verify everything works the same, so we need to use small Merge Requests that focus on individual files or sets of files.