SettingsProvider: allow TETHER_DUN_REQUIRED to be overwritten

Allow overwriting of the default TETHER_DUN_REQUIRED value with
the build property 'ro.telephony.tetherdunrequired'

Recognized values are:
0 for not enforced
1 for enforced
2 for undefined (default; behaves as enforced)

Change-Id: I77c87230b54418ee5c385e5a7effd304a9743990