[11/N] Desks: Implement activation via drag from handle

Wires the drag-to-desktop interaction to move that task to the default
desk and activates that desk at the same time.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 391478992
Test: drag a task to desktop, WM dump shows it reparented to the desk
root

Change-Id: Ia9014a090209ff1b4a1126001d508d59a975f8e2