ProcessDialog, MessageDialog: Support iconed actions

Actions with label+icon and with only icon were displaying wrong before.

* MessageDialog: Just added missing `height: 3.4em;` to enforce height
  when there's no label with its line-height.
* ProcessDialog: Rewritten existing absolute-positioning-based code to
  instead re-use standard button CSS with smaller paddings.

No visible behavior changes for label-only actions.

Bug: T75696
Change-Id: I3c372b5f4c07fd63d9663ba5e03ce50ebe1f55eb
Merge "ProcessDialog, MessageDialog: Support iconed actions"