BC25 Preview current wallpapers (2/2)

Get current wallpapers, home and lock, to display. For the case of 2
different static wallpapers, create 2 different static preview view
models. The view models are created lazily so that if the home and
lock share the same wallpaper, then only create one. Crop hints and
colors are yet to be configured.

Flag: com.android.wallpaper.new_picker_ui_flag
Bug: 348462236
Test: manually verified with combinations of static and live wallpapers
Test: unit tests added
Change-Id: Ibb12a232e7c29b8a9c58744cec7f6f83edee10f7