Fix virtual displays for HWC<=1.1

If we're using a HWC that doesn't support virtual displays, or we have
more virtual displays than HWC supports concurrently, the
VirtualDisplaySurface should simply be a passthrough from source
(GLES) to sink.

This change also tries to distinguish between display types and HWC
display IDs a little better, though there's more to do here. Probably
needs a higher-level rethink; it's too error-prone now.

Bug: 8446838
Change-Id: I708d2cf262ec30177042304f174ca5b8da701df1