Bug 864516 - Refresh time displays only when they are visible

- Made the homescreen, lockscreen and status-bar stop refreshing their
  time displays when the screen is off or they are invisible
- Fixed timer-leaks caused by starting the same timer twice overwriting
  the previous one without cancelling it
- Made the statusbar clock reflect locale changes immediately
Merge pull request #9432 from gabrielesvelto/bug-864516

Bug 864516 - Refresh time displays only when they are visible r=crdlc,yurenju