Desks: Do not move the desk layer back in drag-to-desktop anim

Moving freeform tasks back behind the dragged tasks is needed for actual
desktop tasks with content, but also moving the desk root task ends up
moving the dragged task with it, making it disappear behind the
wallpaper for a few frames.

The root has no content, so there's no need to touch it at all, nor
include it as a "freeform" task.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 406297899
Test: drag to desktop, observe no flicker
Test: atest DragToDesktopTransitionHandlerTest
Change-Id: If7fe221a8f887395b1514e8ee2cf1da8368de854