Initial rename to IpServer

Rename TetherInterfaceStateMachine to IpServer. IControlsTethering
is folded into IpServer.Callback and some of the dependencies in
TetheringDependencies are moved into IpServer.Dependencies.

Several things still need fixing, including:
    - convert message passing into method calls
    - the calls that enable forwarding should be moved up out of
      IpServer into the Tethering layer above it

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
Change-Id: I015f800ed23c8aa5c8c81a74d7b508abfcaab659