[FUI03] No-op refactoring of VpnInfo

As a preparation of exposing system API. This patch does some
harmless refactoring, which includes:
  1. Move VpnInfo to android.net
  2. Add nullability annotations
  3. Make members final

Bug: 174123988
Test: TH
Change-Id: I1530bb45d0e84c28c6deac33596bc686058afc0d