Attempt auto-enter-pip only on user-leaving

Abandon the path to auto-enter-pip in ActivityRecord#makeInvisible and
we should be able to rely solely on TaskFragment#startPausing

Bug: 207739601
Test: ensure regular auto-enter-pip flow works
      && follow the reproduce steps in bug
Test: atest PinnedStackTests
Change-Id: I327e7e84c511100f7a017ad55447bb7f29a763bd