Move VPN routing decisions from iptables to ip

Routes are now encoded by ip rules that send connections to the Vpn
table if the connection is marked and the destination falls into a
route. This differs from the previous design where a mark meant that
the connection must go over the VPN, now a mark simply means that it
may.
Bug: 12549060
Change-Id: I9be7e27a0f46858f109d8bc5c5bced309b05201a