Changed VoicemailProvider demo app to use new VoicemailContract api.

- Use new VoicemailContract instead of VoicemailProvider for API.
  VoicemailContract will soon be submitted to framework/base. The local
  copy will then be deleted.
- Got rid of some unused util/core classes.

Change-Id: Ic7ac76777023a8b94a5ddf2f90caf0bf48d1f60c
13 files changed