Disregard closing activities when blocking previous activities.

- Extending the check added in ag/2468582, if a translucent assistant
  is started on top of a fullscreen activity and subsequently launches
  another fullscreen activity, then returning to the assistant should
  not preclude the otherwise visible fullscreen activity under the
  assistant.  To do that, this logic should only stop checking for
  the visibility of the activities if the task is not finishing and
  will return to another task (otherwise this results in a black
  screen).

Bug: 37527727
Bug: 63581322
Test: android.server.cts.ActivityManagerAssistantStackTests
Test: go/wm-smoke
Change-Id: Ib935b6023b55bda0797f7cb9849595d821ef7d11