1 Previously AuthenticationMethod was a class. Replaced
  it with a simple enum.
2 Removed SharedSecretHash struct and simplified the code
  that passes around PIN hash.
3 If the host config contains PIN in plain text then now
  the host hashes it instead of using spake2_plain
  authenticator method. This means that spake2_hmac is
  always used for Me2Me even when the PIN is not hashed
  in the config.

This change will make it easier to add new authentication
methods.

BUG=589698

Review URL: https://codereview.chromium.org/1755273003

Cr-Commit-Position: refs/heads/master@{#379365}