Convert last FirewallController command to iptables-restore.

This code currently has no callers, but it is the only remaining
user of iptables in FirewallController. Move it to
iptables-restore and delete support for iptables commands from
the class.

Bug: 28362720
Test: unit tests pass
Test: adb shell ndc firewall set_interface_rule rmnet_data0 <allow|deny>
Change-Id: I0a934283ca4479f870139d1ecf90096ae59eb19d