Update color options on wallpaper update (2/2)

Create a callback flow to listen to colors changed events from the
WallpaperManager. Also changed some flows into shared flows to make sure
they are not repeatedly run with each new subscriber.

Flag: com.android.systemui.shared.new_customization_picker_ui
Test: manually verified by changing wallpaper & changing system color
Bug: 288312530
Bug: 388718715
Change-Id: Iab9e802148d0d7e8daffdcee92b521dde2c5bbd6