Allow dumpstate to acquire xtables.lock

iptables recently changed its behavior to strictly require xtables.lock.
dumpstate selinux policy must be updated to allow access.

Bug: 37648320
Test: dumpstate succeeds with no avc: denied ... xtables.lock messages
Change-Id: Ic7e243739f375a60fa14fe67fac910d31d978ffd
(cherry picked from commit ca097979)