Clear all lingering notifications when network is disconnected

In aosp/861571, it tried to refine the design to prevent the
lingering notification is cleared unexpectedly. However, all
notifications should be cleared when network is disconnected.
Refine the design by clearing all lingering notifications when
network is disconnected.

Bug: 123488640
Bug: 123537643
Test: 1.Build pass
      2.atest FrameworksNetTests
      3.Connect to a hotspot which doesn't have internet,
      after getting "NO_INTERNET" notification, disconnect
      the wifi and check if "NO_INTERNET" notification will
      be cleared or not.
Change-Id: Iac45ec186cf81915c318dba37f43136609d579f6

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

@@ -2832,6 +2832,8 @@ public class ConnectivityService extends IConnectivityManager.Stub
         if (DBG) {
             log(nai.name() + " got DISCONNECTED, was satisfying " + nai.numNetworkRequests());
         }
+        // Clear all notifications of this network.
+        mNotifier.clearNotification(nai.network.netId);
         // A network agent has disconnected.
         // TODO - if we move the logic to the network agent (have them disconnect
         // because they lost all their requests or because their score isn't good)