Isolate knowledge that fb target == output buffer

HWComposer didn't allow the virtual display output buffer to be set
directly, instead it always used the framebuffer target buffer.
DisplayDevice was only providing the framebuffer release fence to
DisplaySurfaces after a commit.

This change fixes both of these, so both HWComposer and DisplayDevice
should continue to work if VirtualDisplaySurface changes to use
separate framebuffer and output buffers. It's also more correct since
VirtualDisplaySurface uses the correct release fence when queueing the
buffer to the sink.

Bug: 8384764
Change-Id: I95c71e8d4f67705e23f122259ec8dd5dbce70dcf