GoBackend: set empty underlying networks

https://lists.zx2c4.com/pipermail/wireguard/2020-January/004859.html
https://issuetracker.google.com/issues/114309459
https://developer.android.com/about/versions/pie/android-9.0-changes-all#network-capabilities-vpn

Apparently we need to call this at least once.

Reported-by: Andrey Kupreychik <foxel@quickfox.ru>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/backend/GoBackend.java

@@ -224,6 +224,7 @@ public final class GoBackend implements Backend {
 
             currentTunnel = tunnel;
 
+            service.setUnderlyingNetworks(null);
             service.protect(wgGetSocketV4(currentTunnelHandle));
             service.protect(wgGetSocketV6(currentTunnelHandle));
         } else {