Zoom to Default After ScrollList Item Change in Peek/Single Mode

Currently not resetting the zoom level of the previously current item
in scroll list when changing items via `ScrollList#scrollTo`. Fix.

Unit Tests
-------------

- Added tests for changed code.

How To +10/QA
-------------

```bash
# skip the following steps if you've already initialized the project
$ git clone git@github.com:WebFilings/wf-uicomponents.git
$ cd wf-uicomponents

# do not skip these!
$ git fetch && git checkout zoom_to_default_after_page_change
$ ./init.sh
$ grunt qa
```

- Should see no console errors.
- Open the ScrollList demo page.
- Switch to peek mode.
- Double click the first page.
- Swipe to the second page.
- Swipe back to the first page.
- _Should see first page zoom level reset to default_.

@lancefisher-wf 
@patkujawa-wf 
@robbecker-wf 
@shanesizer-wf