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