Track the companion activity along with companion TaskFragment

When a split pair is created with finish rule, if the primary contains
multiple activities, we will only track and finish the top activity when
the companion TF is finished.

We need to track this info in Core as well to predict back navigation.

Fix: 433459536
Flag: com.android.window.flags.task_fragment_companion_activity
Test: atest WMJetpackUnitTests:SplitPresenterTest
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Test: atest WmTests:BackNavigationControllerTests
Change-Id: Iff00034d3b01a7d719efc031ccf0bcbe34135c92