Your question has already been answered here
Code from the link:
Add this to your styles.xml:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="Theme.DoNotDim" parent="android:Theme">
<item name="android:backgroundDimEnabled">false</item>
</style>
</resources>
And then apply the theme to your activity:
<activity android:name=".SampleActivity" android:theme="@style/Theme.DoNotDim">