Allow multiple adjacent TFs (6/n)

Migrate TaskFragment#getAdjacentTaskFragment for
ActivitySnapshotController and BackNavigationController

Bug: 373709676
Test: refactor, pass existing test
Flag: com.android.window.flags.allow_multiple_adjacent_task_fragments
Change-Id: I45bd8823b19dbdcf659d602f9387bf0dc37b2b22