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

This saves about 100ms on boot.

(cherry picked from commit 546fe48d)

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: I8e15940565894d44a819b9cef25790d443b25df5
Merged-In: I56ce20a0efef8b1aba5f55bc823926447b21a614