[28/N] Desks: Always use prepareMoveTaskToDesk

Migrates callsites still using addMoveToDesktopChanges() to using
prepareMoveTaskToDesk(). The latter is the multi-desk version of the
former that ensures the task is reparented to the destination desk and
skips some windowing-mode changes that are not needed because it doesn't
need to be inherited from the TDA anymore.

In the next CL, these two will be merged into a single function to avoid
repetition and misuse of either one.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: (1) while in desktop, force launch a fullscreen task via adb and
make sure it is reparented to the freeform root when it gets forced into
freeform. (2) move a freeform task to another display and make sure it
is also reparented to the freeform root. (3) same as (2) but moving a
fullscreen task.

Change-Id: Iff6475b6c367d6cdb4215ee067330acc19294f3c