[29/N] Desks: Add #addMoveTaskToDeskChanges

This is a pure refactor. It merges addMoveToDesktopChanges() and
prepareMoveTaskToDesk() into a single method to avoid flag-checking at
different call sites and to prevent misuse of either one.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: atest WMShellUnitTests
Change-Id: I21c3c7c815cacb29e25188fdb9ec9c06382105e3