Bhv 10114 Break down updateMetric() to increase performance

After model added or removed, you should update index bound.
Currently updating index bound logic is included in updateMetric(), so if you want to update index bound then you will undergo redundant work load. 
To increase performance, I pull update index bound logic out of updateMetric()

Enyo-DCO-1.1-Signed-off-by: David Um david.um@lge.com
Hi @DavidUM, there appears to be a Travis error thrown - can we move these functions into `VerticalGridDelegate` instead? Thank you!