BHV-1368: Add wrapper div around ExpandableListItem header.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>
BHV-1368: Add comments re: headerContainer RTL fix.
DCO-1.1-Signed-Off-By: Kevin Schaaf <kevin.schaaf@lge.com>
Merge pull request #1164 from enyojs/BHV-1368-aarontam

BHV-1368: Add wrapper div around ExpandableListItem header.