Move all init code to iptables-restore.

This gets rid of one of the last few uses of iptables, and also
reduces startup time from ~750ms to ~150ms.

Bug: 28362720
Test: bullhead builds,boots
Test: netd_{unit,integration}_test pass
Test: rules after "killall netd" look identical
Change-Id: Idf4d8dbc1292cb0017d4546976ad645a4ac7fa08