seekbar_progress_drawable_shape.xml

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

    <item android:id="@android:id/background">
        <shape>
            <solid android:color="#3700B3"/>
            <corners android:radius="20dp"/>
            <size android:width="200dp" android:height="2dp"/>
        </shape>
    </item>

</layer-list>