Voicemail content provider API for api council review.

This is a new content provider implemented to add visual voicemail
support in android.

Voicemail content provider is the central repository for storing
voicemails inserted by various voicemail sources. The content provider
also exposes a status table to let the voicemail source application
convey its current status to the system.

The primary application that reads from this content provider is the phone app.
The phone app shows voicemails along with other call entries within the
call log and optionally any relevent message about the voicemail source
status.

The implementation of this content provider can be found at
https://android-git.corp.google.com/w/?p=platform/packages/providers/ContactsProvider.git;a=blob;f=src/com/android/providers/contacts/VoicemailContentProvider.java

Change-Id: I8ad46aec20c70684f7bfa45530bbb90dd841d81a