Convert hierarchy to Fragment

[seadowg]

Work towards #5420

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

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?

This shouldn't change any behaviour and the big risk here is that it either does or introduces bugs to flows using the form hierarchy. A general test of using the hierarchy (during form entry and viewing) would be good 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

[seadowg]

@getodk/testers marking this ready for testing before review as it'd be good to get a first pass before I clean things up in case there are some fundamental problems here I've missed.