No-op refactoring of VPN lockdown status check.

Currently, if VPN lockdown is disabled, the blocking judgement
inside VPN will return false immediately. It will make
ConnectivityService hard to check blocked status by a given
VPN lockdown status.

Thus, move this check into ConnectivityService and check it
externally.

Bug: 117814902
Test: 1. manual test with 3rd-party vpn app
      2. runtest frameworks-net

Change-Id: Ia8319b1a1a12f1058c24badf2431f2ec69bc78e7