Add service "gpu" in the SurfaceFlinger process

This service will handle shell commands for querying information about
the graphics hardware and driver. It currently does nothing.

Most shell command services are provided by ActivityManagerService. I
didn't want to put this there, because I'd rather not load graphics
drivers in the system_service process (robustness, security, etc.).
SurfaceFlinger is going to have them anyway, and is the only other
global always-running process that makes sense for this.

Change-Id: I27e451ea62b71aed01c6ececefd76d6707912eed