Clean up invisible tasks in task repo when they are removed

Removing an invisible task is an invisible->invisible change,
so no shell transition runs for this, and DesktopRespository misses
cleaning up task data, which could lead to DesktopTasksController
incorrectly trying to restore the tasks when the desk is
reactivated next time.

This change adds clean-up logic to
RootTaskDesksOrganizer#onTaskVanished() to handle this case.

Flag: com.android.window.flags.enable_desktop_invisible_task_removal_cleanup_bugfix
Bug: 427869488
Test: RootTaskDesksOrganizerTest#testOnTaskVanished_removesChildTask_invokesNonTransitionTaskClosing
Test: DesktopTaskChangeListenerTest#onNonTransitionTaskClosing_invisibleFreeformTask_removesTaskFromRepo
Change-Id: Ic063d287d35a80b7b2abebfa831702232166cf73