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