Add container around the previews in the pin widget sheet.

We did it in past for app-specific picker. Updating the same for the
pin sheet - verified change with Jane Park.

Bug: 379675391
Flag: EXEMPT BUGFIX
Test: Verified with pin sheet and wallpaper colors
Change-Id: I37e7ac93757e95e1940567e800a376256e0b8ed8
diff --git a/res/layout/add_item_confirmation_activity.xml b/res/layout/add_item_confirmation_activity.xml
index d113a38..2bb2eb3 100644
--- a/res/layout/add_item_confirmation_activity.xml
+++ b/res/layout/add_item_confirmation_activity.xml
@@ -71,7 +71,8 @@
                 android:id="@+id/widget_preview_scroll_view"
                 android:layout_width="match_parent"
                 android:layout_height="0dp"
-                android:layout_marginVertical="16dp"
+                android:layout_margin="16dp"
+                android:background="@drawable/widgets_surface_background"
                 android:layout_weight="1">
 
                 <include