5/ Add mixed-type grouped task info

- This will be used to collect multiple grouped tasks
  (ie. split/desktop + fullscreen) into a single grouped task that can
  be used to represent stacked visible tasks in a subsequent CL
- The mixed grouped task is required to have a base grouped task that
  represents the root visible task (generally all the above tasks
  will be visible, but translucent or transient in the case of transitions)

Bug: 346588978
Flag: com.android.wm.shell.enable_shell_top_task_tracking
Test: atest WMShellUnitTests

Change-Id: Id78723e49fa462ac09e16980b418b64234f1da9a