Add a new API to get the network ID range of IPSec tunnel interface

- Add a new API to get the network ID range of IPSec tunnel
interface.
- Use the new API in IpSecServiceTest to make sure the result is
the same. Follow-up commit will change the logic in
IpSecService#reserveNetId(), the modified test can ensure the
correctness of the new change.

Bug: 172183305
Test: atest FrameworksNetTests:IpSecServiceTest
Change-Id: Ic605e48941fc9d6482cdcd01a8adcdc9b6d586a6

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

@@ -17,6 +17,7 @@
 package com.android.server;
 
 import android.annotation.NonNull;
+import android.net.ConnectivityManager;
 import android.util.SparseBooleanArray;
 
 import com.android.internal.annotations.GuardedBy;
@@ -31,7 +32,7 @@ public class NetIdManager {
     // Sequence number for Networks; keep in sync with system/netd/NetworkController.cpp
     public static final int MIN_NET_ID = 100; // some reserved marks
     // Top IDs reserved by IpSecService
-    public static final int MAX_NET_ID = 65535 - IpSecService.TUN_INTF_NETID_RANGE;
+    public static final int MAX_NET_ID = ConnectivityManager.getIpSecNetIdRange().getLower() - 1;
 
     @GuardedBy("mNetIdInUse")
     private final SparseBooleanArray mNetIdInUse = new SparseBooleanArray();