Per vpn setting change in VPN list

- Show admin support details when user taps on a cell and user restriction is on
- Show always-on-vpn active status in preference summary
- User can still open non-configurable per-VPN info page even when user restriction is applied
- Rename ConfigPreference to LegacyVpnPreference
- Move summary String handling into ManageablePreference
- ManageablePreference inherits GearPreference to reuse the code
- Don't show disconnect dialog when always-on is enabled

BUG=26950700

Change-Id: I37b087879cf3b674df528e7787d7bb1eead3310f