BHV-767: Make duration property in videoPlayer works

## Problem
Can not get duration from video player.

## Fix
- Change internal this._duration variable as public this.duration variable.

DCO-1.1-Signed-Off-By: Kunmyon Choi kunmyon.choi@lge.com
Merge pull request #1435 from enyojs/BHV-767-kunmyonchoi

BHV-767: Make duration property in videoPlayer works

Looks good!

Reviewed-by: Aaron Tam (aaron.tam@lge.com)