Quick affordance picker in ThemePicker.

Near end-to-end implementation of everything needed to drive the UI for
the lock screen quick affordance picker/selector in wallpaper picker.

This CL includes:
- A contract file definition (which will be moved into a shared library
  soon)
- A client class to abstract away the content resolver API which, using
  the contract, queries the system UI content provider for data
- A clean architecture implementation with: a repository, an interactor,
  view-models, view-binders, and adapters
- Ugly/minimal, to be replaced and/or updated aggressively when the
  wallpaper team actually implements the UI

Bug: 254857639
Test: Included unit tests, verified manually end-to-end in a temporary
activity in the wallpaper picker build that shows the UI.

Change-Id: Icb492be0041d6a594bdf622c2f3e8177c07b82bf