packages/SystemUI/res/drawable/car_add_circle_round.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="oval">
            <solid
                android:color="@color/car_user_switcher_add_user_background_color"/>
            <size
                android:width="@dimen/car_user_switcher_image_avatar_size"
                android:height="@dimen/car_user_switcher_image_avatar_size"/>
        </shape>
    </item>
    <item
        android:drawable="@drawable/car_ic_add_white"
        android:gravity="center"/>
</layer-list>