app.te: prevent locks of files on /system

Prevent app domains (processes spawned by zygote) from acquiring
locks on files in /system. In particular, /system/etc/xtables.lock
must never be lockable by applications, as it will block future
iptables commands from running.

Test: device boots and no obvious problems.
Change-Id: Ifd8dc7b117cf4a622b30fd4fffbcab1b76c4421b