Let players see which friends are already playing PixelGames.
Players choose who to share with the system picker, so PixelGames only sees the contacts they
select. Both iOS and Android use this picker instead of a blanket address-book permission.
PixelGames matches shared phone numbers or emails to existing players and shows their scores,
unmatched contacts are left alone.
Friends who already play PixelGames show up on the leaderboard; others can be invited.
