[2/N] Use DisplayChange for desk display disconnect.

Rather than routing transitions through the DesksTransitionObserver,
this CL uses disconnectReparentDisplay added to DisplayChange to handle
requests for disconnect transitions in DesktopTasksController.

This has the benefit of allowing shell to make reparenting decisions
before core has a chance to. This means Shell is
largely responsible for the desk disconnect behavior now, which follows
two main rules:

1 - If the destination display supports desktop, reparent all desks to
the new display, with the focused desk on top.
2 - If the destination display does not support desktop, reparent all
tasks in all desks to the destination display and revert them to
FULLSCREEN, with the globally focused task on top.

Since the handling in DesksTransitionObserver is no longer used, it has
all been reverted.

Bug: 391652399
Test: Manual; disconnect a display from both tablet and phone, with
different focused displays. Ensure all above rules apply.
Flag: com.android.window.flags.enable_display_disconnect_interaction

Change-Id: Iaf6c66a70dbec23e7e6abb6d6e737c668087369a