Rewrite DebugInfoActivity to M3

[rfc2822]

(See #691)

I have chosen this Activity as one where not that much happens and I think it could be implemented quite "clean".

To start, I'd:

• [x] Convert M2 calls to M3 (without architectural changes)
• [x] Split into 3 files (Activity, Screen, Model, see #691 and for instance LoginActivity) without further changes
• [x] Change = MutableLiveData() to by mutableStateOf() for our own fields (that don't come from an external Flow) in the model

Then when this is done we can optimize the composable and models a bit and think whether we leave the rest as it is or whether further changes would be useful (like moving the debug info generation to a separate data layer class).

[sunkup]

• [x] Create and use a private set uiState data class in view model
• [x] move remaining screen logic to view model