[Android] Refine the pause/resume behavior for packaged web app

* Pause all timers when the activity is paused, and resume them when the
  activity is resumed.
* Add boolean flag to indicate if the timers are paused or XWalkView is hidden
* Update the API documentations for pauseTimers/resumeTimers are also

BUG=https://crosswalk-project.org/jira/browse/XWALK-1603
Merge pull request #1880 from hmin/timers

[Android] Refine the pause/resume behavior for packaged web app