Try with this :
<?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:centerColor="#555994"
android:endColor="#b5b6d2"
android:startColor="#555994"
android:type="linear" />
<corners
android:radius="0dp"/>
</shape>