BHV-10443 : Moonstone Component Bug for ExpandableListItem.

Bug: Although header text area is tapped with 5-way button, the drawer does not open. 
Fix: SpotlightFocus and tap event handling point are moved from header component to headerContainer component
- Modification is little different with it of 2.5.0 branch (https://github.com/enyojs/moonstone/pull/1378)
- 2.3.0 branch uses image file but 2.5.0 branch uses font character for carat asset

Enyo-DCO-1.1-Signed-Off-By: SoonGil Choi soongil.choi@lge.com