Last week, we had the author of TDD for a Shopping Website LiveProject. Friday at 11am Ranch time, Steven Solomon will be hosting a live TDD session just for us. See for the agenda and registration link
In my opinion, Kotlin will become the standard for Android development within a few years. It is the future of the platform.
You should learn Kotlin if you are going to be an Android developer in a few years. That said, in my experience, the language is not the hard part for any mobile platform. The hard part to learn and understand is the framework. At this moment, I don't think there are many resources that teach you Kotlin and Android development at the same time.
Depending on your comfort level, I'd recommend a hybrid approach. Learn Android development (in Java) and also learn Kotlin at the same time. Perhaps converting what you are learning to Kotlin yourself so that you can see the differences. If you find this to be a little too much, then I would recommend learning Android in Java first and then learning Kotlin.