Nice to see you offer your book here.
Welcome to the Ranch.
Thanks! This is actually my second promotion here. Does anybody remember the first one?
So I've a few questions on Android?
How much time will it take an average Java knowledgeable guy to use Android API?
You can go from zero to your first "Hello, Android" app in a few minutes. Since you already know Java, you know a lot about Android because Android implements most of the Java SE library (see Appendix A, Java vs. the Android Language and APIs). If you know SQL you can use the database, and if you know OpenGL you can do 3d graphics. The trickiest part will be the Android life-cycle, which might throw you for a week or so, and the user interface library, which is almost but not quite entirely unlike Swing and SWT so that could take another few weeks to become proficient in it.
I'm not into gaming stuff, and can i design any gaming applications for Android without much knowledge on Android?
Well if you're just getting started with both gaming and Android you'll have a lot to learn. Creating a fun and engaging game is not easy. Just please don't make another "farting" application.
What is the earning prospects of programming Android? and Is it worth it?
You're not going to get rich any time soon. The Android Market is far younger than, say, the iPhone app store, and the audience is not used to paying for things like iTunes users are. We've all heard the stories of a few people making tens of thousands a day with iPhone apps, and honestly right now on Android nobody is even making hundreds a day. But these are early times, and I believe patience and good work will be rewarded.