VPN settings: update strings for always-on VPN

To conform with UX guidelines and conventions, the following changes are
made to the UI of app VPN's PreferenceFragment:
(1. Always-on toggle title stays the same)
2. Always-on toggle subtext changes from "active"/"inactive" to an
explanation of the feature
3. Lockdown toggle title is shortened
4. Lockdown toggle subtext is removed completely

And the following changes are made to the legacy VPN's ConfigDialog:
5. Error message that is displayed below the "Always-on VPN" checkbox
6. Replacing \u2019 with \'

Bug: 36382729
Test: manually navigate through VPN settings
Test: make SettingsRoboTests
Change-Id: If1b4faec8743dfbf6e3f8062b53190ad63700d25