Program offload-exempt local prefixes into the HAL

Additionally:
    - move mOffloadController into MasterTetherSM

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - observed calls to the HAL setLocalPrefixes in tethering log
Bug: 29337859
Bug: 32163131
Change-Id: Ifaf23c6179ead9de6ccfcf41e0c203025153167b