Restructure use of absent tether_offload_disabled setting

This restructures the fetching of the default disposition such
that we disable (and enable) the feature with only a single
character change.

Additionally: fix unittests with proper use of FakeSettingsProvider.

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passed with developer enabled and disabled
Bug: 29337859
Bug: 32163131
Bug: 63250751

Change-Id: Ib32489d07778465134bca52c589baddbd78ab129