Bug 1054178 - [Gaia][calendar] support 24 hour time format

Because we already defined `shortTimeFormat12`[1], `shortTimeFormat24`[2], `hour-format`[3], and `current-time`[4] in our gaia code base.
[1] https://github.com/mozilla-b2g/gaia/blob/master/shared/locales/date/date.en-US.properties#L202
[2] https://github.com/mozilla-b2g/gaia/blob/master/shared/locales/date/date.en-US.properties#L203
[3] https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/locales/calendar.en-US.properties#L210
[4] https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/locales/calendar.en-US.properties#L212