Stop listening for WiFi interface status changes

Bug: 29931437
Test: Can toggle tethering on and off repeated on shamu and an
      upcoming device, both devices without a SIM.
Test: Can toggle tethering on and off repeated on an
      upcoming device with a T-Mobile SIM.

Change-Id: I4986b1a5f6ae9b45010bd30639806f7ed63a605c