Fix various bugs with model management

- Tie the sound model and keyphrase for simplicity
  We won't support multiple keyphrases in a single model out of the box.
  The db schema will need to be changed by the OEM wishing to add multiple hotwords.
  This is because we currently have no way to test the flow and make sure that things work well with multiple keyphrases
  and also the framework only reads the metadata for one keyphrase.

- Make the delete/update operations atomic

- Make the flow of data from Enrollment -> VIMS; the large sound model doesn't cross the process boundary any other time.
  This is achieved by passing they key around, instead of the model themselves.

- Add a specific delete operation in DatabaseHelper rather than relying on emptying the keyphrases to delete.

Bug: 16555803
Bug: 16516353
Change-Id: I1e0cce137517502a669e431ca7e9f9f755598328