SystemUI IDreamOverlay Implementation.

This changelist implements IDreamOverlay in the form of
the DreamOverlayService. DreamOverlayRegistrant is
responsible for registering this service with the system
as the dream overlay component. This changelist also
introduces a series of interfaces, such as OverlayProvider,
which define the entities DreamOverlayService interacts
with the generate overlay content.

Bug: 201676597
Test: atest DreamOverlayServiceTest
Change-Id: Idb14172672642580fdfa042449b6cad4bf68969b