Chronometer: Update the value only if displayed on screen

Chronometer is refreshed every second even if not displayed
on the screen and if the screen is off.

Change-Id: I9348c4f988b6822f2c7d9cac0e47acaf82a02e01
Signed-off-by: Simon Dubray <simonx.dubray@intel.com>
Signed-off-by: Zhiquan Liu <zhiquan.liu@intel.com>