Add new config key "sshd.kex". The default and supported values are:
1. ecdh-sha2-nistp521
2. ecdh-sha2-nistp384
3. ecdh-sha2-nistp256
4. diffie-hellman-group-exchange-sha256
5. diffie-hellman-group-exchange-sha1,
6. diffie-hellman-group14-sha1
7. diffie-hellman-group1-sha1
With Bouncy Castle installed, all of the above are supported (previously
only 6 and 7). With JCE, only 7 is available.
Bug: Issue 3517
Change-Id: I6b44e88dc4a0ff8f693f21510aba30546bf4cd99
(cherry picked from commit b4a04fa1c5)