Remove background override for sw720dp preference panel

<resources>

    <style name="PreferencePanel">

        <item name="android:layout_marginStart">@dimen/preference_screen_side_margin</item>

        <item name="android:layout_marginEnd">@dimen/preference_screen_side_margin</item>

        <item name="android:layout_marginTop">@dimen/preference_screen_top_margin</item>

        <item name="android:layout_marginBottom">@dimen/preference_screen_bottom_margin</item>

        <item name="android:background">?attr/detailsElementBackground</item>

        <item name="layout_marginStart">@dimen/preference_screen_side_margin</item>

        <item name="layout_marginEnd">@dimen/preference_screen_side_margin</item>

        <item name="layout_marginTop">@dimen/preference_screen_top_margin</item>

        <item name="layout_marginBottom">@dimen/preference_screen_bottom_margin</item>

    </style>

</resources>