Make icon size on Printing settings match the rest of settings

The icons on this page were being displayed at 48dp because we were
using a generic Preference element, instead of the custom settings
AppPreference which we use in lots of other places in settings for
displaying app entries in a list (and has a custom layout that ends up
with 32dp icons).

Fixes: 78654919
Test: manual (go to Settings -> Connected devices -> Connection
preferences -> Printing)

Change-Id: Icf21ab6b41fc00936cd58f3342a8c5502c6dd87f