DeskClock: Redesign ClockFragment

* Use half the width for each, main clock and cities in landscape
* Display the time difference as well for cities in landscape
* Center the main clock when the style is analog, don't do so
  for the digital one
* Merge main_clock_frame to one, no need for separate portrait/landscape
  ones

Change-Id: Icbbf0b74d08a31effb76822232647e03851c8b18