Add network disconnected callback

Create a network callback to notify network agent after the
native network being destroyed by netd which means the network
is fully disconnected. The NetworkAgent may handle this event
after sending disconnect state to ConnectivityService to proceed
its pending works that have to be done after it.

Bug: 178725261
Test: make update-api
Change-Id: I602ff2c688909473b03b72c9407d4286608cff4c
CTS-Coverage-Bug: 178725261
diff --git a/framework/api/system-current.txt b/framework/api/system-current.txt
index c1395d1..593698e 100644
--- a/framework/api/system-current.txt
+++ b/framework/api/system-current.txt
@@ -219,6 +219,7 @@
     method public void onAutomaticReconnectDisabled();
     method public void onBandwidthUpdateRequested();
     method public void onNetworkCreated();
+    method public void onNetworkDisconnected();
     method public void onNetworkUnwanted();
     method public void onQosCallbackRegistered(int, @NonNull android.net.QosFilter);
     method public void onQosCallbackUnregistered(int);