[5/N] Desks: Add DesksOrganizer and create default desks

Adds a DesksOrganizer interface (and root-task based
RootTaskDesksOrganizer impl.) to handle the creation/removal/etc of desk
containers within a display.

As a starting point, when a display appears (and desktop mode is
enabled), an empty desk is created. Additionally,
DesktopDisplayEventHandler is configured to always create a new empty
desk if the display's desk count reaches 0.

The shell command handler createDesk command is also hooked up to create
desks on demand.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 390460212
Test: adb shell dumpsys activity service SystemUIService WMShell | grep
 \ -A20 DesksOrganizer -> shows desk after display appears
Change-Id: Ibe75ce1fbb6dd95a8b53e76d703374ff40d01ebc