Rework OnboardApp to adjust to new architecture

[TimoPtr]

Summary

This PR cleans up the OnboardApp class and keep it only for the wear onboarding since it is not needed for the rest while removing all the useless parameters.

Checklist

• [x] New or updated tests have been added to cover the changes following the testing guidelines.
• [x] The code follows the project's code style and best_practices.
• [x] The changes have been thoroughly tested, and edge cases have been considered.
• [x] Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

Any other notes

I tested the onboarding on TV, mobile and wear.