Use first card for singleton stacks

Singleton stacks had a shadow above them.  That's removed now.
Overscrollmode, migrate divider to padding

Removed the overscrollmode (Google Now and Play have it).  Removed the
4dp transparent divider in favor of card stack padding, which seems
cleaner.
First attempt at view recycling

View recycling works, but only for singleton card stacks.  It pushes too
much work and duplication onto the person implementing the card though.
I'll revamp it to be simpler for the API user.  I'm not using multiple
cards per stack though so I probably won't work on that.  This code is
compatible with multiple cards per stack but it just doesn't optimize
for it.
More view recycling

Card recycling is working well for single-card stacks so far.  Still
need to modify the CardsExample code to work with recycling.