Add a tethering offload settings kill switch

Check for tether_offload_disabled (defaulting to false if not present)
before starting the OffloadController.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
Bug: 29337859
Bug: 32163131
Bug: 34361337
Change-Id: Ifcc0eb1a41f09ad9e717c9252d2d55e69789669b