session-android/res/drawable-v21/media_continue_button_background.xml

<?xml version="1.0" encoding="utf-8"?>
<ripple
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="@color/transparent_white_20">

    <item android:id="@+id/mask">
        <shape android:shape="oval">
            <solid android:color="@color/transparent_black" />
        </shape>
    </item>

    <item>
        <shape android:shape="oval" >
            <solid android:color="@color/signal_primary"/>
        </shape>
    </item>
</ripple>