Memory leak when changing theme/language settings during form entry

[seadowg]

Blocked by #5420

ODK Collect version

v2024.3

Problem description

Changing language/theme settings causes a memory leak as the in memory representation of the form is not cleared away when returning to the main menu on the settings change.

Steps to reproduce the problem

1. Start a form
2. Go to settings and change the language or theme

The memory usage of the app will increase each time you repeat these steps (even after garbage collection).

Expected behavior

The memory increase after opening a form and changing these settings should be cleaned up by garbage collection.

Other information

This happens because we wipe the backstack and start MainMenuActivity again when changing these settings which means that FormFillingActivity doesn't call exit(). This means that the FormController (and other form data) are held in memory indefinitely.

[seadowg]

This was fixed by #6488