Entity validation constraints receive $entity as data

Before #2346373: Data reference validation constraints are applied wrong validation constraints for entities and entity references (such as EntityType and Bundle) received either the Typed data object wrapping the $entity or the $entity object directly - depending on whether the constraint was put on entity level or on an entity reference.

Now, the constraint validator always receives the entity object.

Note: For implementing entity-level constraints see the following change record: https://www.drupal.org/node/2438011