Have registerNetworkAgent return a Network.

This is more useful.

Bug: 135998869
Test: FrameworkNetTests FrameworksWifiTests
Change-Id: I74710b396fa3cec6fbae08d3dcab5bc90dc2e669

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

@@ -575,7 +575,7 @@ public class ConnectivityServiceTest {
                 }
             };
 
-            assertEquals(na.netId, nmNetworkCaptor.getValue().netId);
+            assertEquals(na.network.netId, nmNetworkCaptor.getValue().netId);
             mNmCallbacks = nmCbCaptor.getValue();
 
             mNmCallbacks.onNetworkMonitorCreated(mNetworkMonitor);