Respect user leaving when pausing activity by sleeping

Use another startPausing that passes "mTaskSupervisor.mUserLeaving"
by default. So the user leave hint from starting activity can
propagate to the pausing activity.

This aligns the same behavior as general procedure to pause activity.
So the behavior can restore to as Android T (legacy transition).

Fix: 300390005
Test: atest TaskTests#testUserLeaving
Test: Launch a show-when-locked activity on lockscreen.
      Swipe nav bar to hide it. There should be
      wm_pause_activity with userLeaving=true
Change-Id: I1d73d1ae38c9fa6afc671de63d12025f11f7d07c