Remove BouncyCastle

[MariusVolkhart]

The syncthing-core module declares a dependency on BouncyCastle Mail that we want removed.

It's used for 3 things: Hex encoding, Base64 encoding, and converting the initial client keypair for storage in a Keystore. We have our own Hex already, Base64 is built into Java now, and the key logic should be achievable using only JCA classes. Yes, that would require a Android minSdk of 26, but that's now our main target.