Using the enter/space keys on adjusters inputs submits the Prefs Framework form

Steps to reproduce:

1. Open the UI Options demo
2. Open the show preferences panel
3. Using the keyboard select one of the contrast themes
4. Press the "enter" key
5. Notice that the panel disappears because the form submitted 

It appears the issue is related to FLUID-6539 and the buttons for the switch are triggering the form to submit. This is on the same as FLUID-6507 which submits the form when there is only one text input field.

The solution may be just to update the button type to "button".