[Flexiglass] move getWallpaperColor to Viewbinder

This CL makes sure that getWallpaperColor method is only executed when
binding view to model

Flag: com.android.systemui.scene_container
Bug: 337066000
Test: atest SystemUiRoboTests:MediaControlViewModelTest
Test: atest SystemUiRoboTests:MediaRecommendationsViewModelTest
Test: build, checked UI.
Change-Id: I8422fe3767432ccf41ae3af1910827fb83b04bf7
10 files changed