[FUI03] No-op refactoring of VpnInfo

As a preparation of exposing system API. This patch does some
harmless refactoring, which includes:
  1. Move VpnInfo to android.net
  2. Add nullability annotations
  3. Make members final

Bug: 174123988
Test: TH
Change-Id: I1530bb45d0e84c28c6deac33596bc686058afc0d

core/java/android/net/INetworkStatsService.aidl

@@ -23,11 +23,11 @@ import android.net.NetworkState;
 import android.net.NetworkStats;
 import android.net.NetworkStatsHistory;
 import android.net.NetworkTemplate;
+import android.net.VpnInfo;
 import android.net.netstats.provider.INetworkStatsProvider;
 import android.net.netstats.provider.INetworkStatsProviderCallback;
 import android.os.IBinder;
 import android.os.Messenger;
-import com.android.internal.net.VpnInfo;
 
 /** {@hide} */
 interface INetworkStatsService {

core/java/android/net/VpnInfo.aidl

@@ -14,6 +14,6 @@
  * limitations under the License.
  */
 
-package com.android.internal.net;
+package android.net;
 
 parcelable VpnInfo;