I wouldn't just attach an APK, I suspect few (if any) employers would look at it, and some may even get suspicious of a strange looking attachment and skip over your application entirely.
However, there's nothing wrong with putting a Github link on your CV - if you wish, you can add instructions and an easily accessible APK there.
The only other thing I'd bear in mind is that for anything other than a junior development position / internship, it might look a bit odd if you clearly just created the app for the sole purpose of adding to your CV and nothing else (unless it demonstrated something particularly cool or interesting.)