In addition to updating the remaining English occurrences of "audio call", this
unifies all string constants, localisation comments and UI-level variable names
to use the new "voice call" terminology.
Anything that was obviously protocol-level or related to persistence is still
using the term "audio call", which appears to be a term of art at that layer.
Fixes: https://signalmessenger.atlassian.net/browse/IOS-4850
