Make Vpn.isAvailable return available, not unavailable

Also removed the check about secondary users. We already disable
adding a legacy VPN for secondary users inside the Settings page
itself.

We do not want to stop our users from configuring/disabling third
party VPNs, that's completely supported.

Bug: 23693383
Fix: 32308780
Test: manual, needs robotest
Change-Id: I99ab14b6b26582bdd7fc491c4b2542f3e653f3dc