Bug 931119 - [SMS] Localize labels in thread header, don't hardcode separator character

* Use localizable string to position elements
* Don't hardcode separator characters (' | ', ', ')
* Add 2 tests (different separator characters, fallback for empty separators)
Merge pull request #16983 from flodolo/bug931119_smslabels

Bug 931119 - [SMS] Localize labels in thread header, don't hardcode separator characters