Quick affordance picker in ThemePicker.

Hooks up the UI to the already-existing UI code. We now have a
fully-functional lock screen quick affordance selector experience within
the Wallpaper Picker experience.

Bug: 254858701
Test: Manually verified. Unit tests already exist from previous CLs.
Change-Id: I5688f684a28fb31bd301eaf993d3ac3c79b483e0