Action button improvements:

  - Horizontal layout
  - At most 2 are shown
  - Tombstones are now shown (if the intent is null, the
    button is disabled; use it for quick feedback of an
    action's effect)

Bug: 6418617 (tombstones)
Bug: 6482237 (action separators)
Change-Id: Ie0c613006227bbfe1c0ec6eab1cda4f3782a05f2