SettingsActivity: disable list dividers

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/src/main/res/values/styles.xml

@@ -460,6 +460,7 @@
 
     <style name="PreferenceTheme" parent="FallbackThemingTheme">
         <item name="android:colorBackground">@color/bg_default</item>
+        <item name="android:listDivider">@null</item>
     </style>