Change update machanism more efficiently

Change update machanism more efficiently

[Issue#] N/A
[Problem] This can cause the box' update be starvated.
[Cause] When a box is on background, that box's update timer is just freezed.
[Solution] Even if a box is on background, its timer continues to work.
           And if the timer expires, update is delayed until the box gose to foreground.

Change-Id: Id06683b6133d42678999a223895739443fad353e