Avoid creating IpClient if ethernet cable is unplugged.

This change avoids calling start() on needNetworkFor() if ethernet
cable is unplugged. Fixed by adding isLinkUp validation.

Test: ./cts-tradefed run cts-dev -d -m CtsSecurityTestCases -t \
      android.security.cts.ListeningPortsTest#testNoRemotelyAccessibleListeningUdpPorts
      adb shell netstat -lnup

Change-Id: I2debd7367a20b665249f9c19d2d8a61737435803

service-t/src/com/android/server/ethernet/EthernetNetworkFactory.java

@@ -226,7 +226,7 @@ public class EthernetNetworkFactory extends NetworkFactory {
             }
         } else {
             for (NetworkInterfaceState n : mTrackingInterfaces.values()) {
-                if (n.statisified(request.networkCapabilities)) {
+                if (n.statisified(request.networkCapabilities) && n.mLinkUp) {
                     network = n;
                     break;
                 }