Configuration storage controller should use Entity::id()

The ConfigStorageController::loadMultiple() should use EntityInterface::id() function instead of trying to retrieve a public member. See DatabaseStorageController::loadMultiple() as example.

    // Ensure that the returned array is ordered the same as the original
    // $ids array if this was passed in and remove any invalid ids.
    if ($passed_ids) {
      // Remove any invalid ids from the array.
      $passed_ids = array_intersect_key($passed_ids, $entities);
      foreach ($entities as $entity) {
        $passed_ids[$entity->{$this->idKey}] = $entity;
      }
      $entities = $passed_ids;
    }