ot::Uptime::UptimeToString() method

Converts an uptime value (number of milliseconds) to a human-readable string. The string follows the format "::." for hours, minutes, seconds and millisecond (if uptime is shorter than one day) or "d.::." (if longer than a day). @p aIncludeMsec can be used to determine whether `.` milliseconds is included or omitted in the resulting string.