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

- When enable_shell_top_task_tracking is enabled, we will group visible
  tasks together into a single grouped task that represents the task
  in Overview and in TopTaskTracker.  This allows Launcher to prepare
  overview and gesture nav handling in cases where there are multiple
  stacked tasks (ie. fullscreen/split/desktop + fullscreen)

Bug: 346588978
Flag: com.android.wm.shell.enable_shell_top_task_tracking
Test: atest WMShellUnitTests
Change-Id: I07be12db791b82458d401006c204a11a4384b8aa