split HWComposer out of DisplayHardware

we will only ever have a single instance of HWComposer, so
it's now an attribute of SurfaceFlinger, instead of being part
of DisplayHardware.

DisplayHardware now just represents a "display" (it should be renamed).

Change-Id: Iec191e57686868e1df6daa8b880a286c9fefde56