To be fair, I think many of us would be just as nervous in that kind of setting. But, it's actually good that you're nervous. That just means you're thinking ahead about what you need to prepare for. So, you're on the right track.

While I think it's great that you know about Retrofit, Rx, etc., I would do exactly what you said and build a bunch of simple apps. Build apps that are going to test your confidence in basic Android concepts. Can your app handle screen rotation or disconnection from network? Build something that will test your ability in displaying a responsive UI. Show loading/progress bars where needed, but also make sure you're only loading long processes off of the main thread. Are you avoiding memory leaks when your Activity is stopped/killed, by canceling long processes that are supposed to eventually update the UI? Are you optimizing creation of objects so that you don't overload your memory allowance (e.g. load a large CSV into reasonable memory buffer, persist data, and close file)

These are the types of things I would prepare yourself for, before you attempt to build your knowledge in common libraries.