Re-architext \Drupal\apbs_entities\Allocations::getCustomerId

This is called by \Drupal\apbs_entities\Plugin\EntityReferenceSelection\Animals::buildEntityQuery and \Drupal\apbs_entities\Entity\BaseFieldsTrait::preSave if it's an animal.

The latter should go into the ECA model for animals. The former can probably not call an ECA model at this point. But still, it is a shame that we don't have that logic in ECA so far.