[Provider Model] Remove the background of the title layout

- See b/208220685#comment2 for a screenshot.

Bug: 208220685
Test: manual test and check the UI solution.
Change-Id: I118d16f4a5db32fe975453239f83f58f2e8fa69a

packages/SystemUI/res/values/styles.xml

@@ -837,7 +837,6 @@
     <style name="Widget.SliceView.Panel">
         <item name="titleSize">16sp</item>
         <item name="rowStyle">@style/SliceRow</item>
-        <item name="android:background">?android:attr/colorBackgroundFloating</item>
     </style>
 
     <style name="SliceRow">