base

Switch TLS Channel ID API from ECPrivateKey to PrivateKey.

This is to accept both the "transparent" and "opaque" ECC private
keys. "Transparent" keys provide structured access to their key
material -- these are instances of ECPrivateKey. "Opaque" private
keys are not required to provide structured (or even any) access to
their key material -- these are instances of PrivateKey.

Change-Id: Ib22e18b45b638b429f994ed965416c753226c4ee