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
Merged-In: I8d09e25b83f7ee8be21ec9c9bd3c72a251f1370d
           (cherry-picked from ag/14011912)

tests/net/java/com/android/server/ConnectivityServiceTest.java

@@ -1227,10 +1227,12 @@ public class ConnectivityServiceTest {
             if (mAgentRegistered) throw new IllegalStateException("already registered");
             updateState(NetworkInfo.DetailedState.CONNECTING, "registerAgent");
             mConfig = new VpnConfig();
+            mConfig.session = "MySession12345";
             setUids(uids);
             if (!isAlwaysMetered) mNetworkCapabilities.addCapability(NET_CAPABILITY_NOT_METERED);
             mInterface = VPN_IFNAME;
-            mNetworkCapabilities.setTransportInfo(new VpnTransportInfo(getActiveVpnType()));
+            mNetworkCapabilities.setTransportInfo(new VpnTransportInfo(getActiveVpnType(),
+                    mConfig.session));
             mMockNetworkAgent = new TestNetworkAgentWrapper(TRANSPORT_VPN, lp,
                     mNetworkCapabilities);
             mMockNetworkAgent.waitForIdle(TIMEOUT_MS);