Migrate VPN to the public NetworkAgent API.

On top of being a cleanup this is useful for the S Network
Selection project that will need to enrich the Network
Agent API, and as such should not have to support legacy
agents.

Test: FrameworksNetTests NetworkStackTests
Bug: 167544279
Change-Id: Id3e5f6e19829c64074cd6a52c5f950cee56b860b