Introduce VM injection for home screen and enable screen testing (EXPOSUREAPP-2946, EXPOSUREAPP-2948) (#1320)

* Rename "MainFragment" to "HomeFragment"
Add VM injection.
Move initial logic into VM.

TODO: Move more code, write more tests.

* Additional refactoring, moving logic out of the Fragment.
Moving code into their own encapsulated routines/components.
Use composition to reuse TracingViewModel, SettingsViewModel and SubmissionViewModel

* Introduce @AppContext annotation to prevent anyone mistaking it for something else.

* Add VM to RiskDetailsFragment

* Simplify nested viewmodels, here: TracingViewModel.kt

* Add skeletons instrumentation test skeleton for single fragments with injection and mocking.

* Address PR comments.

* Introduce sealed events class, + some refactoring.

* Make the linter happy.