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: I3fdc4c46675bde48c72424f1cc8f59c3d6b89f0e
3 files changed