[BHV-16348]Issue of co-existing Popup and contextual popup

Issue: If two different popups, both are extended from enyo.Popup exist
at the same time, it creates a conflict behavior as one popup doesn't
have information about the other's z-index.
Fix: zIndex used on different extended popups is made as static property
of enyo.Popup
DCO-1.1-Signed-Off-By: Rajyavardhan P <rajyavardhan.p@lge.com>