ring-android/app/src/main/res/drawable/defaultshadowtop.xml

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

    <gradient
        android:angle="270"
        android:centerColor="#33000000"
        android:endColor="#99000000"
        android:startColor="#00000000" />

</shape>