Fix flaky ConnectivityServiceTest#testNetworkFactoryUnregister

Test: atest ConnectivityServiceTest#testNetworkFactoryUnregister \
      --geneate-new-metrics 100
Bug: 151792032
Change-Id: Iece9e8ad5c51170cd404ac995d0005392d37fe0c

tests/net/java/com/android/server/ConnectivityServiceTest.java

@@ -2465,8 +2465,8 @@ public class ConnectivityServiceTest {
             final MockNetworkFactory testFactory = new MockNetworkFactory(handlerThread.getLooper(),
                     mServiceContext, "testFactory", filter);
             // Register the factory and don't be surprised when the default request arrives.
-            testFactory.register();
             testFactory.expectAddRequestsWithScores(0);
+            testFactory.register();
             testFactory.waitForNetworkRequests(1);
 
             testFactory.setScoreFilter(42);