VPN: reset legacy VPN when resetting IPv4 addresses.

Currently legacy VPN only works on IPv4, and it should always
turn down when the addresses are changed. It assumed that the
interface will be brought down and up, so the event can be
detected via interfaceStatusChanged(). However, the assumption
was incorrect and the event is actually driver-dependent. To
fix this issue, ConnectivityService now tells VPN that the
interface is down when resetting IPv4 addresses.

Change-Id: I76d15e56552d86635c5b274ca980be5da905a6fb