trusty: keymaster: Implement export_key

                                                    const keymaster_blob_t* app_data,

                                                    keymaster_blob_t* export_data) {

    ALOGD("Device received export_key");

    if (error_ != KM_ERROR_OK) {

        return error_;

    }

    if (!key_to_export || !key_to_export->key_material) {

        return KM_ERROR_UNEXPECTED_NULL_POINTER;

    }

    if (!export_data) {

        return KM_ERROR_OUTPUT_PARAMETER_NULL;

    }

    export_data->data = nullptr;

    export_data->data_length = 0;

    ExportKeyRequest request(message_version_);

    request.key_format = export_format;

    request.SetKeyMaterial(*key_to_export);

    AddClientAndAppData(client_id, app_data, &request);

    ExportKeyResponse response(message_version_);

    keymaster_error_t err = Send(KM_EXPORT_KEY, request, &response);

    if (err != KM_ERROR_OK) {

        return err;

    }

    export_data->data_length = response.key_data_length;

    export_data->data = DuplicateBuffer(response.key_data, response.key_data_length);

    if (!export_data->data) {

        return KM_ERROR_MEMORY_ALLOCATION_FAILED;

    }

    return KM_ERROR_OK;

}