Allow multiple adjacent TFs (7/n)

Migrate TaskFragment#getAdjacentTaskFragment for
BackgroundActivityStartController, EnsureActivitiesVisibleHelper, and
LockTaskController.

Bug: 373709676
Test: refactor, pass existing test
Flag: com.android.window.flags.allow_multiple_adjacent_task_fragments
Change-Id: I53b1ac03b0911b663d11ebbb360f617ac04ed50c