Improve cpuset support for surfaceflinger.

SurfaceFlinger needs some of its threads in the system-background cpuset
and some of its threads (the binder pool) outside of the
system-background cpuset in order to improve UI perf/power
consumption. Remove surfaceflinger from the system-background cpuset in
init.rc and allow a thread to place itself in the system-background
cpuset given enough permissions.

bug 25745866

Change-Id: I85f7e41c5439e6ad7cc2d355e51f5dfb3a0c7088