Delete VPN methods in ConnectivityManager.

1. Stop using ConnectivityManager for VPNs in VpnDialogs.
2. Delete updateLockdownVpn, since all callers have been migrated
   to calling VpnManager directly.
3. Delete the call to VpnManager in factoryReset, since the only
   caller (ResetNetworkConfirm) has been updated to call into
   VpnManager directly.
4. Delete getVpnManager, since it is now unused.

Test: m
Bug: 173331190
Change-Id: I5d071281c0e36f6523fea10671a9abf994c66d66