Some public fields are meant to be private
Some static fields are meant to be final
Fix some logics about Story::filterDownloadable

Stories are always downloadable.
Fix some errors in Snap class + new method.

- Fix TYPE_FRIEND_REQUEST_VIDEO_NOAUDIO
- Fix isViewed(); (a screenshoted snap is seen)
- Add isScreenshoted()
Update some Javadoc + add last interaction time.
Variables used for error reporting --> Public
Merge pull request #12 from jamesst20/master

Some public fields are meant to be private and final