Setting/getting per-screen wallpaper (2/3).

Necessary changes to support setting and getting wallpapers per-screen,
which is what's needed for the wallpaper quick switcher to apply the
selected wallpaper only to the screen whose tab is currently selected.

Setting the individual per-screen wallpaper is complete but work still
needs to be done to provide separate recents for each screen. That work
is captured in b/265066284.

Bug: 262924643
Test: manually verified that setting per-screen wallpaper works by
switching between the home screen and lock screen tabs and selecting
different wallpapers
Test: manually verified that undo/restore works as expected, especially
given the lack of per-screen recents to serve as a baseline (it always
resets to both having the same wallpaper)
Test: unit tests throughout the layers

Change-Id: Ib7f312b29476c102621740c2dde8ea8f9b59b55b