Don't add StageChangeRecord when stage task moves to back

We would add a StageChangeRecord for a closing task even when the task
transition change was TRANSIT_TO_BACK. When both of the split stages had
one child task each nothing would happen, but if one of the stages had
two child tasks we would enter logic that assumed that one of stages was
closed. This change adds a StageChangeRecord for a closing task only
when the transition mode is TRANSIT_CLOSE.

Bug: 314688953
Test: atest WMShellUnitTests
Test: Open Gmail and Chrome in split, go to the compose-email screen in
      Gmail, receive call, answer and hang up the call, observe that
      Gmail and Chrome are still paired.
Change-Id: I8ee6c078bd8a1816a96a9515b1a50e652de70d1c