Desks: Create desk (desktop-first) or pre-create root (touch-first)

Moves default desk creation or warm up to happen as a result of TDA-mode
changes rather than display events. Also uses wires the desktop-first
check to RootTaskDisplayAreaOrganizer#isDisplayDesktopFirst to determine
whether to create a full desk or just warm up a root.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 399394443
Bug: 393978539
Test: atest RootTaskDisplayAreaOrganizerTest
DesktopDisplayEventHandlerTest

Change-Id: I323a8fb92186bd86fc0e33d809b89306a63d08ff