[1/N] Desks: Add support for adb commands

Adds support to create/remove desks and manage tasks within them
using adb commands.
This change defines the commands and adds the basic skeleton to handle
them, but doesn't actually implement any functionality.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Test: adb shell dumpsys activity service SystemUIService WMShell
desktopmode <command> <args>
Bug: 389787966

Change-Id: I7f78e77c3ffcec7b47c964a757daa411a0776e52