How hard is it to become a good Android dev?Watch
Good Android developers usually have a few years of commercial experience making apps. They're clued in on good UI patterns on the Android platform and understand when and how to use all the standard SDK components (Providers, Services, Receivers). Hopefully they know better than to suffer from NIH.
Most game development isn't Android specific and won't require deep knowledge of SDK stuff. The logic will mostly be Java or it will run as a native C++ module. Even more likely, it will be platform-independent code with a relatively minimal amount of platform specific work. But a good games developer will have years of experience learning all the standard "things" that go into making games (main loops, AI, animation etc. etc. - it's not my area)
Anyway, as with most things it's not "hard" to become a good developer of any stripe. But there's no substitute for a few years of experience.