Add stackable button bar style

The gravity change caused incompatibility with existing code. So
instead we create SuwGlifButtonBar.Stackable style for the stackable
version.

Test: Manual
Bug: 63399819
Change-Id: I146dbc4f7f2a2019cfe964f59f0a73c099845f30

library/main/res/layout/suw_glif_footer_button_bar.xml

@@ -18,6 +18,6 @@
 <com.android.setupwizardlib.view.ButtonBarLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/suw_footer_button_bar"
-    style="@style/SuwGlifButtonBar"
+    style="@style/SuwGlifButtonBar.Stackable"
     android:layout_width="match_parent"
     android:layout_height="wrap_content" />

library/main/res/values/styles.xml

@@ -211,7 +211,7 @@
         <item name="android:baselineAligned">false</item>
         <item name="android:clipChildren">false</item>
         <item name="android:clipToPadding">false</item>
-        <item name="android:gravity">center</item>
+        <item name="android:gravity">center_vertical</item>
         <item name="android:minHeight">@dimen/suw_glif_footer_min_height</item>
         <item name="android:orientation">horizontal</item>
         <item name="android:paddingTop">@dimen/suw_glif_footer_padding_vertical</item>
@@ -222,6 +222,10 @@
         <item name="android:paddingStart" tools:ignore="NewApi">@dimen/suw_glif_button_margin_start</item>
     </style>
 
+    <style name="SuwGlifButtonBar.Stackable" parent="SuwGlifButtonBar">
+        <item name="android:gravity">center</item>
+    </style>
+
     <!-- Items styles -->
 
     <style name="SuwItemContainer.Description" parent="SuwItemContainer">