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

Btw, what option do you prefer?
* Inside loop
```
 if (isTimeFormatChanged && (!element.dataset.l10nDateFormat12 ||
     !element.dataset.l10nDateFormat24)) {
   return;
 }
```
* or before loop (we don't cover case with empty attributes, but not sure whether it's valid case)
```
var dateTimeElements = document.querySelectorAll(isTimeFormatChanged ? 
  '.l10n-contains-date[data-l10n-date-format12][data-l10n-date-format24]' :
  '.l10n-contains-date'
);
```