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

Change-Id: I015fa3cb05a15624f9e80b7aa5a871cefef431b7