Customizable lock screen affordance entry-point.

Adds an entry-point from the wallpaper picker to a fragment that's
dedicated to show the quick affordance picker. The fragment is currently
just blank.

Bug: 254858701
Test: https://screenshot.googleplex.com/txXtXu86w59HcMB - it shows the
names and icons of the selected affordances correctly and clicking it navigates to
the empty page/fragment

Change-Id: I2487f58aa806333ecf780a607abb7851e63a35e8