Add AuthManager::CreateAccessTokenFromAuth

In case of local auth client passes Macaroon token and device exchanges
that for access token.

BUG=25768507

Change-Id: Ibf126d9ef470cf7843deed6b0b954c99aa64e78d
Reviewed-on: https://weave-review.googlesource.com/2065
Reviewed-by: Vitaly Buka <vitalybuka@google.com>