Wallpaper picker reset support (2/3).

Creates system to support reverting changes made to the home or lock
screen via the wallpaper picker.

The system itself is in WPP2/.../picker/undo.

Bug: 262924056
Test: included unit tests and updated existing ones
Test: manually verified that I can reset changes made to quick
affordances both from the quick affordance screen or the main WPP screen

Change-Id: Id84b27db4264b14436bd73fe6adaac570cffbf37