Support losing the upstream in IpServerTest.

Allow dispatchTetherConnectionChanged to pass in a null
upstream interface and/or null LinkProperties. This will allow
future tests to check what happens when the upstream interface
goes away.

Test: test-only change
Change-Id: I7cfc32d072f23644159d33700d6cd830c426ea19
diff --git a/Tethering/tests/unit/src/android/net/ip/IpServerTest.java b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
index 33b3558..39742f5 100644
--- a/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
+++ b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
@@ -624,16 +624,15 @@
      * @param v6lp IPv6 LinkProperties of the upstream interface, or null for an IPv4-only upstream.
      */
     private void dispatchTetherConnectionChanged(String upstreamIface, LinkProperties v6lp) {
-        mIpServer.sendMessage(IpServer.CMD_TETHER_CONNECTION_CHANGED,
-                new InterfaceSet(upstreamIface));
-        if (v6lp != null) {
-            mIpServer.sendMessage(IpServer.CMD_IPV6_TETHER_UPDATE, v6lp);
-        }
+        dispatchTetherConnectionChanged(upstreamIface);
+        mIpServer.sendMessage(IpServer.CMD_IPV6_TETHER_UPDATE, v6lp);
         mLooper.dispatchAll();
     }
 
     private void dispatchTetherConnectionChanged(String upstreamIface) {
-        dispatchTetherConnectionChanged(upstreamIface, null);
+        final InterfaceSet ifs = (upstreamIface != null) ? new InterfaceSet(upstreamIface) : null;
+        mIpServer.sendMessage(IpServer.CMD_TETHER_CONNECTION_CHANGED, ifs);
+        mLooper.dispatchAll();
     }
 
     private void assertIPv4AddressAndDirectlyConnectedRoute(LinkProperties lp) {