Android - Criar um Button sublinhado

bg_shape.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <gradient
        android:angle="90"
        android:startColor="#f00"
        android:centerColor="@android:color/transparent"
        android:centerX="0.01" />
</shape>

<!--
Para mudar a posição da borda (não necessariamente ser sublinhado)
0 = left
90 = bottom
180 = right
270 = top

Referência: http://stackoverflow.com/a/23754336/1222311
 -->

layout.xml

<Button
android:text="@string/title_btn"
android:layout_width="150dp"
android:layout_height="match_parent"
style="@style/linksActive"
/>

style.xml

<resources>
    <style name="linksActive">
        <item name="android:background">@drawable/bg_shape</item>
    </style>
</resources>