Add default horizontal margin to buttons

These styles are meant to be used automatically by AlertDialog.Builder
in SystemUI. This way, there's margin between the buttons (8dp).

Bug: 203666386
Test: manual (data saver dialog)
Change-Id: I88b7dfb068bba3e655fe0dee2cddfb83674a000e

packages/SystemUI/res/values/styles.xml

@@ -952,6 +952,7 @@
         <item name="android:lineHeight">20sp</item>
         <item name="android:fontFamily">@*android:string/config_bodyFontFamily</item>
         <item name="android:stateListAnimator">@null</item>
+        <item name="android:layout_marginHorizontal">4dp</item>
     </style>
 
     <style name="Widget.QSDialog.Button.BorderButton">