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

samples/VoicemailProviderDemo/src/com/example/android/voicemail/AddVoicemailActivity.java

@@ -130,7 +130,8 @@ public class AddVoicemailActivity extends Activity {
         long duration = durationStr.length() != 0 ? Long.parseLong(durationStr) : 0;
         return VoicemailImpl.createForInsertion(time, sender)
                 .setDuration(duration)
-                .setSource(sourcePackageName)
+                .setSourcePackage(sourcePackageName)
+                .setMailbox(Voicemail.Mailbox.INBOX)
                 .build();
     }