Rework form entry and form hierarchy to be one Activity

[seadowg]

Blocked until v2023.3.x is merged in to master

Currently, there are two Activity objects that handle form entry (answering questions) and viewing/navigating the form hierarchy. This has two major disadvantages:

• Sharing state between multiple Activity objects is far harder than sharing it within a single Activity
• We're unable to embed the hierarchy within a "page" of form entry (like we've seen in some speculative mockups)

We should restructure our code so that FormEntryActivity handles the full form entry workflow. This most likely means creating a two Fragment or View objects that represent different "view" on the form entry session and can easily be switched in between (and potentially embed within each other).

As part of this, we should also remove the FormSessionRepository.

[seadowg]

@grzesiek2010 I'm aiming to convert FormFillingActivity to single Activity/multi Fragment set up with a nav graph (similar to EntitiesBrowserActivity). There will be two fragments: one for form entry and one for the hierarchy. This will allow the form entry view models (mainly FormEntryViewModel itself) to be hosted at the activity level and shared between fragments. Later, we could use the same nav graph to open dialogs (like errors or even things like the select minimal UI), but for this issue I think we just lay the foundations with a simple two destination graph. I'm not sure exactly how the view only mode would work (different nav graph potentially).

After we're done with that, we can review if we can make improvements to how we store the form session. Just moving everything to one Activity here should make managing that state much simpler though.