Add a method to help mocking registerNetworkAgent

This is a trick to help mocking registerNetworkAgent without
having to import its return type. In the near future the
return type will be changed to a non-public class, which
can't be seen in CTS.

Really this shouldn't be necessary because only Connectivity
tests should mock ConnectivityManager, but the practice is
different and other mockers need this support.

See also aosp/3538922

Test: NetworkAgentTest
Change-Id: I3ac63d95d3830a3b413f9ad12b84ac426b63dd08

framework/src/android/net/ConnectivityManager.java

@@ -1242,6 +1242,22 @@ public class ConnectivityManager {
     @ConnectivityManagerFeature
     private Long mEnabledConnectivityManagerFeatures = null;
 
+    /**
+     * A class to help with mocking ConnectivityManager.
+     * @hide
+     */
+    public static class MockHelpers {
+        /**
+         * Produce an instance of the class returned by
+         * {@link ConnectivityManager#registerNetworkAgent}
+         * @hide
+         */
+        public static Network registerNetworkAgentResult(
+                @Nullable final Network network, @Nullable final INetworkAgentRegistry registry) {
+            return network;
+        }
+    }
+
     private TetheringManager getTetheringManager() {
         synchronized (mTetheringEventCallbacks) {
             if (mTetheringManager == null) {