Make the DUN apn data secure.

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