Always-on VPN.

Adds support for always-on VPN profiles. Users pick an always-on VPN
from list of existing VPN profiles, which must use an IP address for
both VPN server and DNS.  Moved "add" operation into action bar.

Bug: 5756357
Change-Id: I4c7ed7f2a3b027be1baf65c08213336a61f3acfe