fix: set new default color for artist folder images

app/src/main/res/layout/item_library_music_index.xml

@@ -15,6 +15,7 @@
         android:layout_height="52dp"
         android:layout_gravity="center"
         android:layout_margin="2dp"
+        android:background="?attr/colorSurfaceVariant"
         app:layout_constraintBottom_toBottomOf="parent"
         app:layout_constraintStart_toStartOf="parent"
         app:layout_constraintTop_toTopOf="parent" />