res/drawable/notification_circle.xml

<?xml version="1.0" encoding="utf-8"?>
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval">

    <solid android:color="?attr/popupNotificationDotColor"/>

    <size
        android:width="@dimen/notification_circle_icon_size"
        android:height="@dimen/notification_circle_icon_size"/>
</shape>