FIx Growing Gap While Peeking

Problem
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While the ScrollList is in peek mode, the content in each item map can move beyond its boundary.

Solution
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Fix the boundary interceptor config while in peek mode.

Unit Tests
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None added

How To +10/QA
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- Travis CI should pass.
- `grunt serve`
- Open ScrollList demo and switch to peek mode.
- Drag the first item such that the second item comes into view.
- _Should not see the gap between items change as you peek._

@lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @tomconnell-wf @todbachman-wf