Fix Peek Mode While Dragging

Problem
-------

Peek mode handling was broken due to events targeting the item map before the list map after removing the ScrollList hit area.

Solution
--------

Put the peek interceptor on the item maps instead of the list map.

Unit Tests
----------

Modified

How To +10/QA
-------------

- 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  
Sanity check - these changes don't change the logic at all, correct?