Add DisplaySurface abstraction

DisplayDevice now has a DisplaySurface instead of using
FramebufferSurface directly. FramebufferSurface implements
DisplaySurface, and so does the new VirtualDisplaySurface class.
DisplayDevice now always has a surface, not just for virtual displays.

In this change VirtualDisplaySurface is just a stub; buffers still go
directly from GLES to the final consumer.

Bug: 8384764
Change-Id: I57cb668edbc6c37bfebda90b9222d435bf589f37