Tethering: Own WiFi tethering state and lifetime

- Add logic to Tethering to track whether the user has requested
  tethering via WiFi.
- Subscribe to intents regarding soft AP state to enable and
  disable tethering when the AP comes up or goes down.
- Refactor IP configuration logic to do configuration for WiFi
  as well as USB.

Bug: 29054780
Test: WiFi tethering continues to work on angler
      Tethering related unittests continue to pass.

Change-Id: I6eff2573ca3fd11fabcf138c468ba517ff2daf65