kcm/automounter: Refactor some methods

To split refactoring and logic changes on following-up
commits

- Use an enum for DeviceModel rows

- Refactor DeviceAutomounterKCM::updateForgetDevice()

- Refactor DeviceModel::rowCount()
- Refactor DeviceModel::index()
- Refactor DeviceModel::flags()
- Refactor DeviceModel::setData()
- Refactor DeviceModel::data()