Correct predictive condition about companion TaskFragment.

There should only close two task fragments together when another task
fragment sets companion to current one. If the bottom tf set it's companion
to top tf, that means the bottom tf will be close while top activity
be finished.
Ref JetpackTaskFragmentOrganizer#setCompanionTaskFragment

Bug: 325504342
Test: atest BackNavigationControllerTests
Change-Id: I1dfb9ed8b27a520db0a240e2645910656d1894df