Use Elliptic Curve encryption instead of RSA

This should fix SFTP not working when using GSConnect (which doesn't specify the SSH parameters we do to allow old cyphers).

Requires API 23, so on pre-23 we still use RSA. This means that it won't be possible to pair an Android API23+ with an Android API22 or lower. Doing the same change on the other clients can wait until we do drop support for pre-23, since that isn't causing any problems.