Refine alpha animation for the preview pager (2/2)

Use a shade to cover the preview until the preview is ready to show.
This can avoid flashes from the live wallpapers.

Test: Manually tested. See bug.
Bug: 404918868
Flag: com.android.systemui.shared.new_customization_picker_ui
Change-Id: I6bfafaa0712fe3d69c90ebfc53128781d0331f98