Not dynamically add system-background tasks to bg_schedboost_fd.

System-background tasks can be either in bg_schedboost_fd or fg_schedboost_fd.
It is better to set the right cgroup via init.rc and not dynamically in
set_cpuset_policy.

BUG: 29533997
BUG: 29512132
Change-Id: If5dab78eafbe2d0d03fd7fd3f36d794cb4a7645f

libcutils/sched_policy.c

@@ -302,7 +302,6 @@ int set_cpuset_policy(int tid, SchedPolicy policy)
         break;
     case SP_SYSTEM:
         fd = system_bg_cpuset_fd;
-        boost_fd = bg_schedboost_fd;
         break;
     default:
         boost_fd = fd = -1;