Fix Tethering settings.

Two issues.  A mcc/mnc-driven overlay means that the config at boot may not be
the config we wish to use - the sim card is read later which may switch the
config.  Changed to read the configuration each time rather than once at boot.
Second, the secure-setting override was always trumping the resource config
as we weren't discriminating between a not-set default and a real setting.
This meant the config could never make DUN-required.

bug:5495862
Change-Id: Icd4e90ac1d32bbb704c0ff9cc69e954fb0a0b58c