Use RouteInfo API constructor in VpnService

Instead of using the RouteInfo(destination, gateway) constructor which
is private API, use the RouteInfo(destination, gateway, iface, type)
constructor which is system API.

This separates the VpnService better from internal implementations, and
is necessary to move RouteInfo to the connectivity module.

Bug: 174436414
Test: m

Change-Id: Idc98b2258f3c78bdba40b5fcb97769b5ad99d016