Make FirewallController::createChain use replaceUidChain.

Make FirewallController::createChain use replaceUidChain.

This has two benefits:

1. It makes the behaviour of setting firewall chains via the
   firewallReplaceUidChain RPC match the behaviour of creating
   the chains on boot. (As a side effect, it reduces code
   duplication between the two.)
2. It makes creating firewall chains on boot use iptables-restore,
   which is substantially faster than running iptables commands
   one at a time.

This CL will allow the framework to switch to using
firewallReplaceUidChain when the framework starts, providing
substantial speedups over the current behaviour of running two
iptables commands for every app that is whitelisted or idle.

Bug: 26675191
Change-Id: Ifbd15bf9143efd526570dde8f88effc79d164630