Android :: Working With Apis/Libraries

I'm building an app where I want to architect the app layer separately from the service layer (not android services). So I'll create a service interface(the apis) and then create a class(es) that implement that service interface. Inside the android application layer, I want to code to just that service interface. I think this is similar to how the google maps apis works. the api interface and stubs are in the framework, then each device actually provides the implementation. I don't need to go that far, but I am wondering how I package this up. One, where would I put these interfaces, so I can code to them in the app layer (like Activities). Second, how do I tell Android where the implementing classes are? I'm guessing this is somewhere in the manifest? For now I think I would probably just build the service implementation with the application source code, if that's easier. Eventually I'd like to break that out and drop it in as a 3rd-party jar.