empathy: Don't count inactive players to advance from answering phase

This commit implements the desired behavior described in the previous
commit message. The test case that was added in that commit now
passes, (as does the entire test suite).

In the implementation here, we need a count of all player that have
the active property set to true. Instead of looping over the array and
counting this, we have the Game class track this count as players are
added and removed.