Setting/getting per-screen wallpaper (1/3).
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: I6514247977e497547c4421be6663db84217175cd
19 files changed
