Support to close two TaskFragments in predict back animation

...via CompanionTaskFragment.

TaskFragment#getCompanionTaskFragment provide a more reliable signal
to predict whether two TaskFragment close together.
When currenttask fragment has set companion task fragment, back predict
should searchfor next activity below the bottom task fragment, or
predict to previous task if no more activity exists.

Bug: 311336844
Bug: 303614109
Test: atest BackNavigationControllerTests
Test: launch two task fragment and set them as companion, verify each
case separate,
When stacked: predict should show next activity below those two task
fragment, or show previous task if no more activity.
When adjacent: predict should do nothing if there is more activity
below the task fragment, or show previous task if no more activity.

Change-Id: I1451432f041fa1f2f7f76a865bdd6823edf66c10