Add TLS Channel ID support to SSLCertificateSocketFactory.

This adds a new method setChannelIdPrivateKey as a hidden API.

See http://tools.ietf.org/html/draft-balfanz-tls-channelid-00 for
more information about the TLS Channel ID extension.

Change-Id: I73c1905afdce01d4831de7faa55ea4496575b5a5