ENYO-4067: Center floating popup relative to full viewport.

Issue
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Floating popups that have `centered:true` set are centered relative to their instance owner instead of the full viewport.

Fix
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Position centered, floating popups relative to the full viewport, otherwise position relative to instance owner.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>