Push DNS configuration on network validation

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
    - connected to captive portal network,
      saw failed validation attempt (opportunistic),
      passed portal, saw successful revalidation attempt
Bug: 64133961
Bug: 72344805
Bug: 109928338

Change-Id: I0471685c2234af8a8c7f5b2ad3205eb6e36333c5