Video player: fix start-end time bugs.

This PR fixes following bugs ([BLD-468](https://edx-wiki.atlassian.net/browse/BLD-468)):
* start time doesn't work in the flash mode;
* speed does not change if changed before video starts playing;
* end point for the videos with small length is built incorrectly;
* slider has incorrect position, after video stops playing.

@valera-rozuvan , @jmclaus , please review.