refactor: merge AgendaLayoutViewModel with AgendaPagerViewModel and access it only from the top-level AgendaLayout

[cbeyls]

This allows loading and caching all the Agenda data (rooms, sessions, speakers) in a single StateFlow in a single ViewModel instead of loading the rooms twice.

• It follows the best practice of making reusable Composable functions more modular by decoupling them from the ViewModel. AgendaViewModel is now only retrieved and observed from a single place: the top-level AgendaLayout, and callbacks are used in all lower-level composables.
• Deleted the obsolete AgendaLayoutViewModelTest as it brings no real value. More useful tests should be written in the future.