Genericize promo card.

+ Make ViewHolder generic to generic to promo cards with customized
text or graphic layouts, as long as they implement two actions,
now designated the primary and secondary action views. This resuses
code which is logically similar between promo cards, and hopefully
doesn't bite us back in the future if there is a divergence in promo
card structure.

+ Make the GroupingListAdapter slightly "safer".

Bug: 23422274
Change-Id: I60bced4dbc61fec2e225a1cae9726ac40ed679a5