Move NetworkStack to services.net

NetworkStack is only used in services.net or clients of services.net. It
cannot stay in framework.jar because it needs to depend on AIDL
interfaces, which would conflict with app implementations if they were
in framework.jar.

(cherry-pick of aosp/905233 with trivial conflicts in SystemServer.java)

Test: atest FrameworksNetTests NetworkStackTests
Bug: 124033493
Change-Id: I501b125a388c1100c2182bde4670944c2f0d7a02

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

@@ -88,6 +88,7 @@ import android.net.NetworkQuotaInfo;
 import android.net.NetworkRequest;
 import android.net.NetworkSpecifier;
 import android.net.NetworkStack;
+import android.net.NetworkStackClient;
 import android.net.NetworkState;
 import android.net.NetworkUtils;
 import android.net.NetworkWatchlistManager;
@@ -5135,7 +5136,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
         if (DBG) log("registerNetworkAgent " + nai);
         final long token = Binder.clearCallingIdentity();
         try {
-            mContext.getSystemService(NetworkStack.class).makeNetworkMonitor(
+            getNetworkStack().makeNetworkMonitor(
                     toStableParcelable(nai.network), name, new NetworkMonitorCallbacks(nai));
         } finally {
             Binder.restoreCallingIdentity(token);
@@ -5147,6 +5148,11 @@ public class ConnectivityService extends IConnectivityManager.Stub
         return nai.network.netId;
     }
 
+    @VisibleForTesting
+    protected NetworkStackClient getNetworkStack() {
+        return NetworkStackClient.getInstance();
+    }
+
     private void handleRegisterNetworkAgent(NetworkAgentInfo nai, INetworkMonitor networkMonitor) {
         nai.onNetworkMonitorCreated(networkMonitor);
         if (VDBG) log("Got NetworkAgent Messenger");