[16/N] Desks: Create minimization root for each desk

Creates an additional root task for each desk to host minimized desktop
windows. Also adds DesksOrganizer#minimizeTask to move a task into it.

The minimized root is top-level (child of the DefaultTDA) for now, and
uses WCT#setHidden to always remain invisible.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 391485148
Test: put tast into desktop, then minimize it, verify WM hierarchy is as
expected

Change-Id: I00c0ff07760e65508e19fea5733899e81c1f9cd7