Remove the shadow in the bookmark as it doesn't have title.

Fix http://b/issue?id=2158516
diff --git a/res/values/styles.xml b/res/values/styles.xml
index 21ab98f..fa29a2a 100644
--- a/res/values/styles.xml
+++ b/res/values/styles.xml
@@ -28,6 +28,7 @@
 
     <style name="BookmarkTheme" parent="@android:Theme.Black">
         <item name="android:windowNoTitle">true</item>
+        <item name="android:windowContentOverlay">@null</item>
     </style>
 
     <style name="AutoCompleteTextView">