add property to adjust extra_free_kbytes kernel vm tunable

ActivityManager can't directly write to extra_free_kbytes because
/proc/sys rejects all chown and chmod syscalls.  Proxy the writes
through init by using the sys.sysctl.extra_free_kbytes property.

Bug: 10024467
Change-Id: I441e00478421254355fcafb252bc878166483d4c
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 3990287..c7e9690 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -402,6 +402,10 @@
 on property:sys.powerctl=*
     powerctl ${sys.powerctl}
 
+# system server cannot write to /proc/sys files, so proxy it through init
+on property:sys.sysctl.extra_free_kbytes=*
+    write /proc/sys/vm/extra_free_kbytes ${sys.sysctl.extra_free_kbytes}
+
 ## Daemon processes to be run by init.
 ##
 service ueventd /sbin/ueventd