Use FileObserver for photo callback.

Try out with this gist https://gist.github.com/scottgwald/8312821. Performance is fast and robust using cameraPhotoPath.  In my tests just now, I got the callback in between 4 and 7 seconds 5 times in a row. This seems a lot faster than it was with the polling method. Not sure why that would be unless for some reason the photo writing was happening on the same thread? Anyway, I'm psyched!