make choice_field_mask work in embedded admin as well

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Until now, this special form type only worked on stand alone admins, but not in embedded admins as created by sonata_type_collection