I know from SSLHandshakeException: Handshake failed on Android N/7.0 and my own experience that this curve is not available in the system crypto lib for Android up to 7.1.2 (though it is available in Android web browsers such as Chrom{e,ium} or Firefox and its derivative, as you can read in Android - SSL/TLS and ECC (Elliptic curve cryptography) for instance).
My question is whether it was added on Android 8.x or not, and in this latter case do you have any idea why it is not supported?
While I have no definitive answer regarding Android 8.0 Oreo, I can definitively say that X25519 is available for KEX in Android 8.1 after having tested it.