[SMS App] Add i18n application domain for messaging app

## Summary

Add KLocalizedString::setApplicationDomain for kdeconnect-sms

Per the programmer guide, this must be done, otherwise translations will not happen.
https://api.kde.org/frameworks/ki18n/html/prg_guide.html

## Test Plan

### Before:
kdeconnect-sms would spew errors like `kf.i18n: KLocalizedString: Using an empty domain, fix the code. msgid: "%1" msgid_plural: "" msgctxt: ""`
(Translations still seem to work, though)

### After:
No more ki18n errors, translation works.