[8/N] Desks: Implement move-to-desk for running tasks

Replaces #moveTaskToDesktop with #moveTaskToDefaultDeskAndActivate for
existing callers, and adds a new #moveTaskToDesk to enter desktop mode
with a task on a specific desk.

The adb command |moveTaskToDesk| is wired to this new method.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 391478992
Test: adb shell dumpsys activity service SystemUIService WMShell
 \ desktopmode moveTaskToDesk <task> <desk>
Test: enter desktop mode from handle menu works

Change-Id: I345317e531696e2d0ea93727e35bda58ebf37a61