Add test coverage for IptablesAlert{,Fwd}Cmd.

Additionally, remove some unused code.

(cherry picked from commit 615df791)

Bug: 37641280
Test: marlin builds and boots
Test: new unit test passes
Test: netd_{unit,integration}_test pass
Change-Id: I8224b4cc0382f5efe57723baa1513c693d42535b
Merged-In: I32072a2701fe1f52d5b3cfb0d57b3f296d7c37df