[FUI04] Refactor VpnInfo

As a preparation of exposing system API. This patch does some
harmless refactoring, which includes:
  1. Change raw arrays into lists according to API guidelines.
  2. Write test.
  3. Rename class to UnderlyingNetworkInfo.
  4. Rename vpnIface to iface.
  5. Make underlyingIfaces @NonNull in order to adapt new
     unparceling code.
  6. implement equals and hashCode for testing.

Test: atest android.net.UnderlyingNetworkInfoTest
Bug: 174123988

Change-Id: I405c21e57c4af8a12a9dd0a1749b9e6690f87045