[6/N] Desks: Implement desk removal

Wires DesktopTasksController#removeDesk to remove the desk root task
using a shell transition, and updates the repository state when the
transition completes.

Also wires the desktop command handler to remove a desk through adb.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 390460212
Test: adb shell dumpsys activity service SystemUIService WMShell | grep
 \ -A20 DesksOrganizer -> does not show desk after removal
Change-Id: I3618950c6483efc20bec9e0e0cbd0dcf1ac04307