Remove touch effect of Ratio on listitem

Remove the ripple touch effect of RatioButton,
at Network ratings provider settings page.

Change-Id: Iba067450901572492df68974bdfc14c9ef09a655
Fixes: 70818673
Test: Manual test

res/layout/preference_widget_radiobutton.xml

@@ -21,5 +21,6 @@
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:layout_gravity="center"
+    android:background="@null"
     android:focusable="false"
     android:clickable="false" />