Add VcnTunnel, UnderlyingNetworkTracker skeletons

This change adds skeletons for each of the various major components used
in the VCN.

Additionally, this change adds the VcnGatewayConnectionConfig, used to
configure each logical connection to the VCN gateway. Each
VcnGatewayConnectionConfig specifies a Network that can be brought up on
demand for a given telephony service (INTERNET, MMS, DUN, etc)

Bug: 163602123
Bug: 163432852
Test: Skeletons only, no major testable pieces
Change-Id: I7297690573c1b6ebd0ab9c1d434dd45c1e7d841d