VPN settings: add missing string resource

Previously, a string resource was deleted by mistake in commit
cce4e612. This bug went uncaught by
local and presubmit build checks. We add the missing string back to
strings.xml to fix the build. The affected string is referenced in
ManageablePreference.java.

Apologies for any inconvenience.

Test: make SettingsRoboTests; manually navigate through VPN settings
Bug: 36815998
Change-Id: Ibc2ef9ad43cad73ef5f078714faf5c52bc7198c6