Builds approval object urn.

def BuildApprovalUrn(self, approval_id):
    """Builds approval object urn."""
    event = rdf_events.AuditEvent(
        user=self.token.username,
        action="CLIENT_APPROVAL_REQUEST",
        client=self.subject_urn,
        description=self.reason)
    events.Events.PublishEvent("Audit", event, token=self.token)

    return self.ApprovalUrnBuilder(self.subject_urn.Path(), self.token.username,
                                   approval_id)