Initial code for listing/storing sound models

- We use a DB to store and persist the sound models.
- This'll be used by SoundTriggerModelManager, a service that lists,
  deletes and registers new models. This'll be used by the enrollment
  client to enroll and unenroll users.

- This needs the unique identifiers for sound model & keyphrases to be
  present in the respective data structures

This is very early stage so please point out any concerns.

Change-Id: I82962895bf326167458f20e6ba995295551de025