Most secure peer-to-peer exchange of public keys? Just what the question asks: I would like to have two Android users exchange public keys for use in a mobile app while not connecting to a centralized hosted server or website.
Public Keys are PUBLIC: they are meant to be shared. So, I think it wouldn't matter how you transfer those keys. What is more important is how strong those keys are encrypted, so they aren't easily hacked or bypassed.

You might also want to check on these two questions:


Strong encryption for base64 encoded public key in Android
how to secure Android Market public key