Bug 1035762 - [B2G][SMS] support 24 hour time format. r=schung

Earlier here was ```messageL10nDateFormat``` property too (as far as I understood it was used by code that was triggered on ```languagechange``` to manually update date string), but it was removed in https://github.com/mozilla-b2g/gaia/commit/685fdbdce0488b5de381f9260e1799efbcd4d5a2. Can you remember why it was removed?