SSL: select the right key type for kx
SSL: select the right key type for kx

During the switch to BoringSSL this function was rewritten and it
requested DH public key with RSA signature for a lot of things.

(cherry picked from commit d8606d56b6367d55174527c2206e51b474caf0d2)

Bug: 20641394
Change-Id: Id3880b01ed1810c5d7af9996c48ce45fdf4850f8

google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt