shadow.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:startColor="#33000000"
        android:endColor="@android:color/transparent"
        android:angle="-90">
    </gradient>
</shape>

YourLayout.xml

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
        <!-- Your views here -->
        <View
            android:layout_width="match_parent"
            android:layout_height="8dp"
            android:background="@drawable/shadow" />
    </RelativeLayout>