Begin moving VPN to NetworkStateTracker pattern.

Created base tracker that handles common bookkeeping, and move VPN
to become a tracker.  VPN status is now reflected in NetworkInfo, and
is mapped to LegacyVpnInfo.

Legacy VPN now "babysits" any init services it starts, watching for
when they stop unexpectedly.

Bug: 5756357
Change-Id: Iba7ec79da69469f6bd9a970cc39cf6b885b4c9c4