Added support for RootTaskDisplayAreaOrganizer

Allows shell to have information on the parent container for tasks so it
can do things like attach other surfaces to it. E.g. split-screen
divider.

Also, have DisplayAreaOrganizer ctor take an executor so incoming calls
from the binder threads can be posted to the main thread.

Bug: 175416931
Test: They pass!
Change-Id: Ia3d0b978cf43badd8c495b75708aa027fe9a2aa1