Use nav graph in form hierarchy

[seadowg]

Work towards #5420

As much as possible, this prepares FormHierarchyFragment to be used in a single activity with a (not yet ready) FormEntryFragment in a nav graph.

Why is this the best possible solution? Were any other approaches considered?

I thought I'd already done this as part of an earlier refactor, but just realized it was missing. The next step here is to convert FormFillingActivity into a host activity a single destination nav graph (FormEntryFragment) and then we'll be able to integrate FormHierarchyFragment.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Navigating to and from the hierarchy is the area most affected here.

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines