Checks if user has access to a client under given URN.

def _HasAccessToClient(self, subject, token):
    """Checks if user has access to a client under given URN."""
    client_id, _ = rdfvalue.RDFURN(subject).Split(2)
    client_urn = rdf_client.ClientURN(client_id)

    return self.CheckClientAccess(token, client_urn)