Rather than come out of the user-modifiable APN DB, the DUN APN data will come first from a built-in resource and then potentially overriden by a secure setting (which is gservices upgradable). Also made the "require-dun" setting secure-setting overridable. bug:2736390 Change-Id: I1e4644c3839f06c977b83797641f3948785146a2