BHV-15102: Display SCRIM when popup is shown

**Issue:** 
When multiple popup(having animation) are shown and hidden alternatively, the SCRIM behind the popup is hidden. The last popup which was made hidden, triggers the animation end event which hides the SCRIM.
**Fix:** 
SCRIM is made visible when the popup showing animation is complete.

Enyo-DCO-1.1-Signed-off-by: Anish Ramesan <anish.ramesan@lge.com>
Actually, is this line even needed? The super-kind is also making the same call in `showingChanged`: https://github.com/enyojs/enyo/blob/master/source/ui/Popup.js#L471