Make taskbar visibilty change only in desktop.

Make the DesktopVisibilityController not change the taskbar visibility
when there are no freeform tasks visible.

Test: Go from freeform to fullscreen, observe taskbar doesn't pop up
Bug: 323129412
Flag: ACONFIG com.android.wm.shell.enable_desktop_windowing DEVELOPMENT
Change-Id: I67f246cfc2ddc198326c093fd0af83886de8b0a9