Data stall detection using DNS event

If dns resolver on a network get consecutively timeout then it
is a strong signal that the network is no longer usable.
Reevaluate the network once it's data stall suspected

Test: 1. runtest frameworks-net
      2. SettingsBackupTest passes
      2. Run on wifi w/o internet capability
Bug: 112653893, 113916551

Change-Id: I74287b174d933f97a91fa1529b1809856ac3b38d

services/core/java/com/android/server/ConnectivityService.java

@@ -1659,6 +1659,24 @@ public class ConnectivityService extends IConnectivityManager.Stub
                 loge("Error parsing ip address in validation event");
             }
         }
+
+        @Override
+        public void onDnsEvent(int netId, int eventType, int returnCode, String hostname,
+                String[] ipAddresses, int ipAddressesCount, long timestamp, int uid) {
+            NetworkAgentInfo nai = getNetworkAgentInfoForNetId(netId);
+            // Netd event only allow registrants from system. Each NetworkMonitor thread is under
+            // the caller thread of registerNetworkAgent. Thus, it's not allowed to register netd
+            // event callback for certain nai. e.g. cellular. Register here to pass to
+            // NetworkMonitor instead.
+            // TODO: Move the Dns Event to NetworkMonitor. Use Binder.clearCallingIdentity() in
+            // registerNetworkAgent to have NetworkMonitor created with system process as design
+            // expectation. Also, NetdEventListenerService only allow one callback from each
+            // caller type. Need to re-factor NetdEventListenerService to allow multiple
+            // NetworkMonitor registrants.
+            if (nai != null && nai.satisfies(mDefaultRequest)) {
+                nai.networkMonitor.sendMessage(NetworkMonitor.EVENT_DNS_NOTIFICATION, returnCode);
+            }
+        }
     };
 
     @VisibleForTesting