Fix subclassing and stop overriding constructors in advagg_cdn\Form\SettingsForm

Problem/Motivation

Add object that uses the FactoryInterface should have dependencies inject via ::create and not ::__construct(). This allows parent classes to change their constructor without breaking any code.
@see https://events.drupal.org/seattle2019/sessions/drupal-9-coming-getting-y...

Example: \Drupal\config_sync\Controller\ConfigSyncController::create
@see https://www.youtube.com/watch?time_continue=1554&v=hN9KjaBvAUk
@see https://www.previousnext.com.au/blog/safely-extending-drupal-8-plugin-cl...

Proposed resolution

Fix subclassing and stop overriding constructors.

Related issue #3106647: Fix tests that broken with changes in advagg_cdn\Form\SettingsForm