Switch to use SHA-256 for certificates / fingerprints.

This CL changes identity generation to use SHA-256 for the self-signed
certificates and the fingerprints sent in the SDP.

BUG=4602
R=juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/47149004

Cr-Commit-Position: refs/heads/master@{#9173}