Program offload-exempt local prefixes into the HAL

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - observed calls to the HAL setLocalPrefixes in tethering log
Bug: 29337859
Bug: 32163131
Merged-In: I3c3bd4ee83ffe86ddbe6a16fbbfa0756bf5064d4
Merged-In: Iaf80e584df458010741401fc1a19e7f6e21923c1
Merged-In: Ifaf23c6179ead9de6ccfcf41e0c203025153167b
(cherry picked from commit 32179ff8)

Change-Id: I65ea833ac6fe7a75ebe546cfa358c4cffdf2de05