BHV-13623: Prevent ListActions from being covered by moon.Scroller.

Issue
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In specific instances where overlapping content is supposed to float above a `moon.Scroller`, the content is not actionable.

Fix
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In the case of `moon.ListActions` in a medium `moon.Header`, we adjust the z-position of the drawer so that it is in the same plane as `moon.Scroller`'s strategy.

Note
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We may need to consider coming up with something similar for other "floating" controls that can be actionable, such as `moon.Popup` (see this fiddle from the community: http://jsfiddle.net/L4477/).

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>