Adds settings item for quick affordances.

This is in Display > Lock screen. It reads "Buttons" and the summary
text below it is a comma delimited list of the names of the
currently-selected quick affordances.

Fix: 256662519
Test: Manual verification that the lock screen and wallet
items are gone and the new item is visible and clicking it opens the
Wallpaper & style settings screen

Change-Id: If3746b5d0eb8c61edb9378cdb217ca248b999944