Rename VoLTE/WFC enable API to available - add VT available API. [DO NOT MERGE]

Renamed API to use consistent semantics: available refers to the current
state of the service rather then whether feature is enabled by device/carrier/
user.

Added VT API to match VoLTE/WFC.

Change-Id: Ifb31f123db77e731e75a982b8594168f59a289e7