Revise lockdown check in getLegacyVpnInfo()

Return null instead of raising exception in lockdown mode.

BUG=22749146

Change-Id: I0e9e3ded0d189e3fa9e872b7af9fe363799c85d6

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

@@ -3048,7 +3048,10 @@ public class ConnectivityService extends IConnectivityManager.Stub
     @Override
     public LegacyVpnInfo getLegacyVpnInfo(int userId) {
         enforceCrossUserPermission(userId);
-        throwIfLockdownEnabled();
+        if (mLockdownEnabled) {
+            return null;
+        }
+
         synchronized(mVpns) {
             return mVpns.get(userId).getLegacyVpnInfo();
         }