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