external: Bump stoken to 0.81-rc

The fix in 0.81 shouldn't really affect Android, but let's make sure it
doesn't break it either.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
screenshots: Add feature graphic

This isn't referenced anywhere in the source repo, but it is used on the
Google Play listing.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Update changelog and bump version to v0.91-final (919)

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>