Merge IPv6TetheringInterfaceServices into TetherInterfaceStateMachine

Ideally this would have been done immediately after IPv6 tethering code
was made part of an official named release. Getting this in now should
help with the relaying of LinkProperties that is required for offload.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - runtest frameworks-wifi passes
Bug: 29337859
Bug: 32163131

Merged-In: Ib6e6356e6caa469523fc098e850108de97b494e2
Merged-In: Iec52309a56dd38ac9854ce40a792b3b622f0b015
Merged-In: I5eb3c02331aabc033ecfebee2900477c61e1b6f6
Change-Id: I015fa3cb05a15624f9e80b7aa5a871cefef431b7
(cherry picked from commit 94ae4c99)