Replace all member status strings with MailchimpLists constants

Closes #3409177

modules/mailchimp_lists/mailchimp_lists.module

@@ -16,6 +16,7 @@ use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\field\Entity\FieldConfig;
+use Mailchimp\MailchimpLists;
 
 /**
  * Implements hook_entity_delete().
@@ -463,7 +464,7 @@ function mailchimp_lists_get_subscribers(FieldConfig $field, &$context) {
 
   $mc_list_id = $field->getFieldStorageDefinition()->getSetting('mc_list_id');
 
-  $matches = mailchimp_get_members($mc_list_id, 'subscribed', $options);
+  $matches = mailchimp_get_members($mc_list_id, MailchimpLists::MEMBER_STATUS_SUBSCRIBED, $options);
 
   if ($matches) {
     if (!isset($context['sandbox']['max'])) {