Adjust color logic to be based only on home wallpaper

Previously, the default wallpaper colors is based on the latest
wallpaper. Update it to be based only on the home wallpaper.

Flag: com.android.systemui.shared.new_customization_picker_ui
Test: manually verified
Test: ThemeOverlayControllerTest
Bug: 376522054
Change-Id: I459c65477c24b18d076a34acadb645ffe75f0e7d