Use artwork theme color for background and scrim

This makes the scrim match the artwork colors, plus fixes background
color when there is no artwork and/or when device is in dark mode

Flag: com.android.systemui.media_controls_in_compose
Bug: 397989775
Test: verify appearance in dark and light mode, with and without art
Change-Id: I495997cc3314d2cd4f761c2418228d2b0437cc48