Centralized Dream Overlay State.

This changelist introduces DreamOverlayStateController,
a singleton for managing dream overlay state. The controller
currently maintains the collection of overlays (by way of
DreamOverlayProviders) to show over a dream. Entities, such
as the DreamOverlayService, can register for updates to the
collection.

Bug: 201676403
Test: atest DreamOverlayServiceTest
Test: atest DreamOverlayStateControllerTest
Change-Id: I096dbea4b99fbbe7f5ec74d954db5b8af80ef68b