Fix Peek Mode While Dragging

Problem
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Peek mode handling was broken due to events targeting the item map before the list map after removing the ScrollList hit area.

Solution
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Put the peek interceptor on the item maps instead of the list map.

Unit Tests
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Modified

How To +10/QA
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- Travis CI build should pass.
- `grunt serve`
- Navigate to the ScrollList demo.
- Set the fit mode to `width` and the scroll mode to `peek`.
- Grab the first item in the list and drag it up and down over and over at different speeds. Drag it enough to reveal the second item in the list.
- _Should see smooth dragging behavior without the content jumping._
- _Should see first item snap back to bottom of viewport if releasing while peeking less than 1/3rd the viewport height._
- _Should not see any inconsistency in the gap height between the first and second item._

@lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @tomconnell-wf @todbachman-wf  
https://github.com/WebFilings/wf-uicomponents/pull/74/files#diff-3e3b3e374c73172cba0356146b5e542cR71