This week's book giveaway is in the Agile and Other Processes forum. We're giving away four copies of Real-World Software Development: A Project-Driven Guide to Fundamentals in Java and have Dr. Raoul-Gabriel Urma & Richard Warburton on-line! See this thread for details.
Short answer: of course -- in fact, since Android apps are written in Java, you can't be an Android developer without being a Java developer.
Longer answer: you're never just one type of developer -- at least not if you want to be a good one. And, before diving off into Android, I'd advise getting a good grounding in Java. Otherwise, you're going to get in over your head very quickly.