Video player: speed change bug

This PR fixes [BLD-424: Speed does not change if changed before video starts playing](https://edx-wiki.atlassian.net/browse/BLD-424) .

**Description**: The speed can only be changed when the video is playing. Changing the speed while the video is paused (stopped, or initially loaded) has no effect on playback rate, but the speed menu shows the changed speed.

@valera-rozuvan please review.