Fetch DISALLOW_CONFIG_VPN for the current user.

The code did not specify the user in which to fetch the value
of the restriction DISALLOW_CONFIG_VPN. Since it was called
from the vpn service which lives in user 0, it would always fetch
the value for user 0.

BUG: 18902920

Change-Id: I89419976a8edcaa86ac8e545c64d10818cd42ddd