Send add/removeDownstream info to offload HAL

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passed
Bug: 29337859
Bug: 32163131

Merged-In: I0cb81ac054fc2bf6c8b8bfe658e9404a15091d7a
Merged-In: I7abcdcc2d7d967179c47081a6db2b417164891f3
Change-Id: I6c59aa7cb80b54f376f294b24c1409710c553d74
(cherry picked from commit ed962a84)