[SettingsLib] Update the expandable preference button's shape

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item>

        <shape

            android:shape="oval">

            android:shape="rectangle">

            <size android:width="32dp" android:height="40dp"/>

            <corners android:radius="@dimen/settingslib_expressive_space_large1"/>

            <solid android:color="@color/settingslib_materialColorSurfaceContainer"/>

        </shape>

    </item>