Previously PairingAuthenticator classes were responsible for handling
client ID. This meant that in case there are multiple pairing
authentication methods the client would have to resend the client_id
if the host chooses different authentication method. Now the client
ID is handled by the negotiating layer, so client_id needs to be sent
only once.

Also now NegotiatingAuthenticators create PairingAuthenticators even
when falling back to PIN. This ensures that <pairing-failed> tag is
always present when falling back to PIN.

BUG=593123

Review URL: https://codereview.chromium.org/1781173005

Cr-Commit-Position: refs/heads/master@{#381633}