Internal API for system apps to determine default network for other apps

Callers with CONNECTIVITY_INTERNAL permission can read off the netId
that an application is assigned to by default.

Necessary for making connections as the default network for a client
app eg. when downloading stuff on its behalf.

Bug: 27074270
Change-Id: I8d35e8e99126875f55f3c545090326f3e9be43fb

core/java/android/net/ConnectivityManager.java

@@ -766,6 +766,28 @@ public class ConnectivityManager {
         }
     }
 
+    /**
+     * Returns a {@link Network} object corresponding to the currently active
+     * default data network for a specific UID.  In the event that the default data
+     * network disconnects, the returned {@code Network} object will no longer
+     * be usable.  This will return {@code null} when there is no default
+     * network for the UID.
+     * <p>This method requires the caller to hold the permission
+     * {@link android.Manifest.permission#CONNECTIVITY_INTERNAL}.
+     *
+     * @return a {@link Network} object for the current default network for the
+     *         given UID or {@code null} if no default network is currently active
+     *
+     * @hide
+     */
+    public Network getActiveNetworkForUid(int uid) {
+        try {
+            return mService.getActiveNetworkForUid(uid);
+        } catch (RemoteException e) {
+            throw e.rethrowFromSystemServer();
+        }
+    }
+
     /**
      * Configures an always-on VPN connection through a specific application.
      * This connection is automatically granted and persisted after a reboot.

core/java/android/net/IConnectivityManager.aidl

@@ -44,6 +44,7 @@ import com.android.internal.net.VpnProfile;
 interface IConnectivityManager
 {
     Network getActiveNetwork();
+    Network getActiveNetworkForUid(int uid);
     NetworkInfo getActiveNetworkInfo();
     NetworkInfo getActiveNetworkInfoForUid(int uid);
     NetworkInfo getNetworkInfo(int networkType);