Adding voicemail transcription state to the database

The transcription-state column will be used to provide better visual
feedback for users when viewing voicemails.  Specifically it will
help disambiguate the cases of transcription-in-progress and
transcription-unavailable.

For now the new public API constants are marked @hide.

This database change involves 2 related cl's, one to define the new
column and one to update the database schema (ag/2129944, ag/2129909)
The cts test will be updated when the column is made public.

Test: run cts -t android.provider.cts.contacts.VoicemailContractTest -m CtsProviderTestCases
Bug: 37340510
Change-Id: I6fd5f52a81f43afda425227394f3e571c072b18f