Support model deletion

the updateKeyphrase call is also used to clear out a sound model currently,
this happens when the sound model passed in has no keyphrases.

We can revisit if we need another deleteSoundModel method

- Also fix an issue with the way we were writing keyphrase to the DB
  and when updating keyphrase, we actually addOrUpdate

Change-Id: Ib7250c2fdafef6bc40387912a79366c334d73292