Use RootTDAOrganizer to obtain TDA windowing mode in desktop mode

For the time being, we only change the root TDA's windowing mode, so it
is OK for us to use the root TDA's windowing mode to resolve the final
windowing mode request in desktop mode.

Bug: 333724879
Test: Enter/exit desktop mode still works. The task windowing mode isn't
overriding its container's.
Test: atest DesktopTasksControllerTest

Change-Id: I9469b597445efa4dc75c0fc8e2d48ba0e4c6f937