6b/ Group visible tasks into a single mixed grouped task

- Update to use mixed grouped tasks for visible tasks (if there are more
  than one).  This allows us to remove/reframe some existing assumptions
  about which task in the visible list is what the user is considering
  the base task in Overview

Bug: 346588978
Flag: com.android.wm.shell.enable_shell_top_task_tracking
Test: atest WMShellUnitTests
Change-Id: I283a342e623e7f7a4c5a4440c97867cbeec47f18