Update VpnManagerServiceTest to work with lockdown VPN main user changes.

Bug: 259515323
Test: atest VpnManagerServiceTest
Change-Id: If56a9994bc89c09bedc8709e5b9db871372f7418

tests/unit/java/com/android/server/VpnManagerServiceTest.java

@@ -131,6 +131,11 @@ public class VpnManagerServiceTest extends VpnTestBase {
                 Vpn vpn, VpnProfile profile) {
             return mLockdownVpnTracker;
         }
+
+        @Override
+        public @UserIdInt int getMainUserId() {
+            return UserHandle.USER_SYSTEM;
+        }
     }
 
     @Before