HWComposer now has its own concept of display IDs

HWComposer can now create IDs representing a display
it can deal with. IDs MAIN and HDMI are reserved.
SurfaceFlinger associate HWComposer IDs with a
DisplayDevice and uses that when it talks to HWComposer.

A DisplayDevice doesn't have to have a HWComposer ID,
in that case it just can't use h/w composer composition.

Change-Id: Iec3d7ac92e0c22bf975052ae2847402f58bade71