Move runIptablesAlert{,Fwd}Cmd to iptables-restore.

This saves about 100ms on boot.

Bug: 37641280
Test: marlin builds and boots
Test: netd_{unit,integration}_test pass
Test: iptables rules look identical to other marlin running oc-release
Test: Enabling/disabling tethering adds/removes the forward rule
Change-Id: I56ce20a0efef8b1aba5f55bc823926447b21a614