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

framework/api/module-lib-current.txt

@@ -115,10 +115,12 @@ package android.net {
   }
 
   public final class VpnTransportInfo implements android.os.Parcelable android.net.TransportInfo {
-    ctor public VpnTransportInfo(int);
+    ctor public VpnTransportInfo(int, @Nullable String);
     method public int describeContents();
+    method @NonNull public android.net.VpnTransportInfo makeCopy(long);
     method public void writeToParcel(@NonNull android.os.Parcel, int);
     field @NonNull public static final android.os.Parcelable.Creator<android.net.VpnTransportInfo> CREATOR;
+    field @Nullable public final String sessionId;
     field public final int type;
   }