Cleanup. Changing voicemail_transcription_enabled to voicemail_transcription_available

Follow the convention that features allowed by server configuration flags use the
word 'available' and when a feature is allowed by the user via settings then use the
word 'enabled'

Bug: 37340510
Test: manual and updated unit tests
PiperOrigin-RevId: 178831467
Change-Id: I5691fd7db45a70908d32cdac2b049fe76e6a0680
4 files changed