[38/N] Desks: Disable launch-adjacent when a desk is active

To prevent a launching task with the launch-adjacent flag set from
launching outside of desktop mode, this change disables the
launch-adjacent container temporarily while the desk is active.

This is the multiple-desks version of what FreeformTaskListener did
before, but because desk roots (freeform root tasks) aren't managed by
FreeformTaskListener, the handling needs to be moved into
RootTaskDesksOrganizer. Also because a desk could be empty, so relying
on freeform task count doesn't work anymore.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 400495830
Test: Use Chrome's "New Window" to launch an adjacent instance - verify
it launches inside the desk.

Change-Id: I6bbba8e50dc2f596ac5a9f1ef18b404a5af4863c