IDreamOverlay Introduction.

This changelist introduces IDreamOverlay, an interface
for rendering an overlay on top of a Dream. IDreamOverlay
implementations can register with DreamManagerService to
be bound to by DreamService when dream commences. The
overlay will be passed the Dream Window information so
that it may render within this window space.

Bug: 201676854
Test: atest DreamOverlayTest
Change-Id: I382ba2acb4a6922f19250753f8f0afc4181a6454