vpn2: show third-party VPN services

VPN apps are shown alongside configured VPNs now. The requirement that
a password is set is now only enforced when setting up a configured
VPN as this is not necessary for apps.

Some UI redesign.

Bug: 19573824
Bug: 17474682
Bug: 19575658
Change-Id: I02bd977136929647d65b9784fb4cc5df24b45428