You should use the public key for your application. As described here, you can find it in the details for the application under "Services & APIs".
Regarding testing with GenyMotion, I can't speak to that directly, as I haven't used it. But from what I've read, I have my doubts. Certainly you can't use it out of the box; it does not include a recent version of Google Play, which is required for testing in-app billing. I found this blog post on how to install Google Play, but I have no idea if it will actually work.