As someone who enjoys answering questions under the java tag, it becomes quite annoying when the java page becomes filled with questions about Android. It's starting to seem like java is simply a throw-in tag when asking a question about Android.
The android tag is for questions which deal with that operating system. The java tag is for questions either about that language or with code in that language. The two are neither mutually exclusive nor dependent on each other. Many questions can and should have both applied -- questions which deal with features that are specific to that OS, from the perspective of code. Some, although asked by someone programming on Android could use java and leave android off, because the OS is irrelevant. A question which doesn't feature code, or isn't about the language, shouldn't have java.
I'm completely unfamiliar with Android, but if there's a primary framework (on iOS there is Cocoa Touch) that might also be a candidate for tagging in many cases.
There are five tag slots available on each question, and it's rare, in my experience, to need more. Tags shouldn't be used willy-nilly -- only those that truly apply should be there -- but any and all that do helpfully categorize the question should be present. So, yes, if there's Java in the question, it should have java, even if it's also about that stupid mobile OS.
*And so I share your frustration, as someone who loves ObjC questions but is tired of the hundreth iteration of "how can i rotate my tableview in ios objective-c" since I don't really work with iOS. I am ruthless about editing objc off of questions where it does not apply.
I have a habit of retagging platform questions with the language where applicable, although I haven't done that with android questions for a long time. Admittedly this is only most helpful on questions about platforms that support multiple languages; I'll tag a question with the language to make it easier to tell at a glance what language the asker is coding in.
That said, you can always add android to your ignored tags and questions with that tag will be hidden anywhere you're looking at a list of questions.