Fix empty desktop mode after closing the last window

- Added `closingTasks` to repository to keep track of the tasks that are currently closing but not yet vanished.
- Checking for closing and minimized tasks in #isOnlyNonClosingTask
- Back navigation handled in DesktopTasksController#handleBackNavigation and closing task is cleaned up in onTaskInfoChanged.

Bug: 343726497
Test: atest WMShellUnitTests:DesktopTasksControllerTest WMShellUnitTests:DesktopModeTaskRepositoryTest
Flag: com.android.window.flags.enable_desktop_windowing_wallpaper_activity
Change-Id: I3a8ba46df3f97bb6f32283500f84d2ebb3ebe2fe