Make interface IP serving code set LinkProperties

Additionally, clean up awkward IPv6TetheringInterfaceServices
instantiation mechanics.  In future, this class will be absorbed
by TetherInterfaceStateMachine (prior to its renaming to IpServer).

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

Change-Id: Ib620e3df182f9f8e2c019aae1cd8981eb0b29376