MailchimpListsSubscription doesnt save 'subscribed' in DB

Problem/Motivation

I wanted to check a MailchimpListsSubscription Entity for the 'subscribed' value through using
$subscription->getSubscribe()
Strange enough it always returned NULL. Although the checkbox seemed to save the changes when I checked/unchecked it.

The problem was that, during building the form/widget, the code checked the DB first and immediatly after checks mailchimp for the subscription and sets the default value for the field.
mailchimp_d8/modules/mailchimp_lists/src/Plugin/Field/FieldWidget/MailchimpListsSelectWidget.php::formElement():33

    // Check the current Instance / read from DB
    $subscribe_default = $instance->getSubscribe();

    $email = NULL;
    if (!empty($instance->getEntity())) {
      $email = mailchimp_lists_load_email($instance, $instance->getEntity(), FALSE);
      if ($email) {
        // Check mailchimp itself
        $subscribe_default = mailchimp_is_subscribed($instance->getFieldDefinition()->getSetting('mc_list_id'), $email);
      }
    }

It is possible that there are 2 different states on Mailchimp and in the DB which should be checked for. But thats another problem.

The actual problem that getSubscribe() doesn't work is because of the extra ['value'] array dimension in several places, so the field in the DB never gets actually updated.

Proposed resolution

Using the ['values'] property of the Instance of the Class correctly.
Removing the extra ['value'] dimension where necessary.

Remaining tasks

I havent tested this thoroughly, but it doesn't seem like a big change.
Also, as seen in the patch, the property value ['interest_groups'] was affected. It threw an error at me while viewing the field settings of a 'Mailchimp Subscription' field. Not anymore.

User interface changes

API changes

Data model changes