Reset $source_field after event call in MediaEntityDropzoneJsEbWidget::submit

Problem/Motivation

In MediaEntityDropzoneJsEbWidget::submit, the media entity can be altered in
$event = $this->eventDispatcher->dispatch(Events::MEDIA_ENTITY_CREATE, new DropzoneMediaEntityCreateEvent($media_entity, $file, $form, $form_state, $element));.
This can result in a change in the name of the source field. But while $media_entity is set anew after this call, $source_field is not.

Proposed resolution

Set $source_field anew.

Remaining tasks

User interface changes

API changes

Data model changes