(work in progress) Update for LLAccordionCtrl arrange single expansion.

Found this issue while working on EXT-6564(Fix wearable editing panels) - Every single_expansion accordion is treated like "fit_parent" is set to "true" after changes made in 9805 : e6c5f3b23be1 (fix for EXT-5128 Groups accordions should all appear on-screen w/out scrolling (open accordion should fill space)).
Updated accordion to take into account "fit_parent" property.

Reviewed by Vadim Savchuk - https://codereview.productengine.com/secondlife/r/147/

--HG--
branch : product-engine