Skip overriding windowing mode with multi-desks enabled

This CL prevents desktop-first switching logic to update the override
windowing mode of the existing freeform tasks if multi-desks is enabled.
With multi-desk backend, freeform tasks are no longer the direct child
of TDA so no need to override it.

Bug: 391965153
Flag: com.android.window.flags.enable_multiple_desktops_backend
Flag: com.android.window.flags.enable_display_windowing_mode_switching
Test: DesktopDisplayModeControllerTest
Change-Id: Ifd86e531c01e2e647f57d0585b34ce9bde79f9e4