Use mem cgroups in libprocessgroup if they're available

Use mem cgroups if the kernel is built with CONFIG_MEMCG=y,
additionally add system group.

test: verified on both kernel with/without memcg enabled
Change-Id: Ia2ae89efa3905e9da68fa77adac1225c667864d1
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 84d0f87..d3a1d17 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -36,6 +36,8 @@
     mount cgroup none /dev/memcg memory
     # app mem cgroups, used by activity manager, lmkd and zygote
     mkdir /dev/memcg/apps/ 0755 system system
+    # cgroup for system_server and surfaceflinger
+    mkdir /dev/memcg/system 0550 system system
 
     start ueventd