Manage VPN legacy state separately from NetworkInfo

This is mainly a cleanup, but is also necessary for the
network selection project.

This is for network selection ultimately because NetworkSelection
needs NetworkAgents to use the newer API introduced in R rather
than the legacy internal API. Using that API forbids communicating
to ConnectivityService through NetworkInfo, and does not support
the FAILED state because there is no usage in connectivity.
In VPN, FAILED is used only to communicate a state to Settings
and it does this through IConnectivityManager.getLegacyVpnInfo,
which already is using an int to communicate this information.

Splitting the legacy state from NetworkInfo not only is simpler
ultimately because it's the format in which it's consumed, but
also will allow removing NetworkInfo completely.

Test: FrameworksNetTests NetworkStackTests
Bug: 167544279
Change-Id: I8b95e020919e38a5166892221096db6271985574