Moved playback init to loadedmetadata handler for proper seeks

Video object's `setCurrentTime` should not be called before `loadedmetadata` event.
That leads to `seek` not working when called from `Stream.load`.

The patch moves playback initialization to `loadedmetadata` event handler. Now `seek` works fine.

With live streaming that makes big sense since client should not play live stream from zero time but from the first fragment currently available.