NatController: Fix tethering-counting iptables rules handling

Fix duplicate tethering-pair detection. It was broken because the
underlying mechanism used quota2 which has a 15char limit and
the two combined interface names were longer than that.

Fix parsing or tether-counting rules when no interfaces are available
vs when parsing is broken.
Now the parser is not invoked if no tether-counting rules were created.

Bug: 10710027
Change-Id: I37899d113a37cd84255d439efa6e2ed3ce712ec0
5 files changed