Allow DependencySerializationTrait to work with readonly properties

🐛 Error: Cannot initialize readonly property Drupal\og\Plugin\Field\FieldWidget\OgComplex::$membershipManager Active We are running into this on OG, thanks for putting this issue together @alexpott

Using Drupal\Core\DependencyInjection\DependencySerializationTrait should not make it impossible to have readonly properties that refer to services on a class or classed that inherit from it.

Would this work?

• Change line 81 to (new \ReflectionProperty($this, $key))->setValue($this, $container->get($service_id));
• Change line 96 to (new \ReflectionProperty($this, $key))->setValue($this, $entity_type_manager->getStorage($entity_type_id));