4a/ Track visible running tasks via transitions observer

- When enable_shell_top_task_tracking is enabled, use the existing
  task transition observer (until we have the generalized task repo)
  to track the set of visible tasks for dispatching to Launcher.
  In addition, update the paths to compose GroupedTaskInfos instead
  of raw RunningTaskInfos so that we can later coalesce tasks that
  should be bundled together for presentation in Launcher (or whose
  joint states are required by launcher).

Bug: 346588978
Flag: EXEMPT adding new flag enable_shell_top_task_tracking
Test: Build SystemUI & Launcher
Test: atest WMShellUnitTests
Change-Id: I12bb44ff8ccc07709cb3a2aa44e853fe0cc7ea45