Add "close" socket option for immediately close socket after finish check captive portal.

Unexpected packets are observed after captive portal check. Due to the packets, modem can't fall-back to idle state (modem remained as active state).
It makes unnecessary power consumtion.
To prevent it, "close" option is added in sendHttpProbe()

Test: check socket close.
Change-Id: I5b6987071ad915081846504adbb209eb371fd1c3

packages/NetworkStack/src/com/android/server/connectivity/NetworkMonitor.java

@@ -1312,6 +1312,7 @@ public class NetworkMonitor extends StateMachine {
             urlConnection.setInstanceFollowRedirects(probeType == ValidationProbeEvent.PROBE_PAC);
             urlConnection.setConnectTimeout(SOCKET_TIMEOUT_MS);
             urlConnection.setReadTimeout(SOCKET_TIMEOUT_MS);
+            urlConnection.setRequestProperty("Connection", "close");
             urlConnection.setUseCaches(false);
             if (mCaptivePortalUserAgent != null) {
                 urlConnection.setRequestProperty("User-Agent", mCaptivePortalUserAgent);