Make nested adapter classes private

[martinbonnin]

Follow up from #2924 (review)

Making nested adapter classes private doesn't quite work at the moment because ResponseFields "jump" class nesting:

class Data_Adapter {
  companion object RESPONSE_FIELDS = arrayOf(ResponseField(fieldSets = listOf(
    // Cannot access 'HumanCharacterHero': it is private in 'Hero'`
    FieldSet("Human", Hero.HumanCharacterHero.RESPONSE_FIELDS)
    FieldSet(null, Hero.OtherCharacterHero.RESPONSE_FIELDS)
  ))

    private class Hero {
       private class HumanCharacterHero {...}
       private class OtherCharacterHero {...}
    }  
}

Generating the list of FieldSets in Hero up there would most likely fix this