Make generic BaseNetworkObserver.

This makes it easier to add new INetworkManagementEventObserver events
in future.

Change-Id: I432263d745558de4b878f313c1951230ed9db2b1

services/java/com/android/server/net/NetworkStatsService.java

@@ -763,7 +763,7 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
     /**
      * Observer that watches for {@link INetworkManagementService} alerts.
      */
-    private INetworkManagementEventObserver mAlertObserver = new NetworkAlertObserver() {
+    private INetworkManagementEventObserver mAlertObserver = new BaseNetworkObserver() {
         @Override
         public void limitReached(String limitName, String iface) {
             // only someone like NMS should be calling us