Add session ID to VpnTransportInfo

- This will be visible only to apps with the NETWORK_SETTINGS
  permissions (signature), and will be redacted for all other callers.
- This string is expected to be the same as set by
  VpnService#setSession, and in general, VpnConfig.session. But it
  will be a general API that Vpn.java can call when setting the
  VpnTransportInfo.
- This string cannot be updated once the VPN NetworkAgent is connected.

Bug: 171872481
Test: atest ConnectivityServiceTest
      atest VpnTransportInfoTest
      atest android.net.cts.NetworkAgentTest
Change-Id: I8d09e25b83f7ee8be21ec9c9bd3c72a251f1370d