Fix empty desktop mode after closing the last window (part 2)

- Changed `isOnlyActiveNonClosingTask` to `isOnlyVisibleNonClosingTask`, because back navigation doesn't remove task from active and doesn't call `onTaskVanished`
- Removed closing tasks removal from the `onTaskVanished` as it's handled in `onTaskInfoChanged`
- Changed `handleBackNavigation` to `handleTaskClosing`, because app can decide to close on back navigation and won't get handled

Bug: 343726497
Test: atest WMShellUnitTests:DesktopTasksControllerTest WMShellUnitTests:DesktopModeTaskRepositoryTest
Flag: com.android.window.flags.enable_desktop_windowing_wallpaper_activity
Change-Id: Ia4a8d8f1dbaf25536cb470a4501e899c61845f69