Android: Better Unread Counting

I believe this fixes the many issues that were causing #460.  Not only were we sometimes double-decrementing unread counts when a story was viewed, we were also sometimes skipping them entirely.  Worse yet, we were often resetting the counters on context switch.  This fixes all of those, and more.

This approach is a little bit more heavyweight, though, in order to gain accuracy.  These changes should be tested on a few devices before we push them, just in case.