Make the border of PreviewIcons transparent

The border had a solid color before. This works for the Gallery example
but we have transparent and blured backgrounds in Notifications now.

The border was too thin before, increased to 2.5.dp

Bug: b/418022243
Test: Screenshot test
Flag: com.android.systemui.notification_bundle_ui
Change-Id: Ic59582be0dda7e4562c11df9e501db2525240546