Bugfix/seekable not updated for mp4 sourcechange

[wjoosen]

This PR fixes two issues

1. An issue where due to the lack of progress events firing (as expected) for MP4 assets, the seekable state is not updated if you switch to an MP4 source on the same player instance. This messes up the SeekBar, since the thumb is rendered at a percentage into the timeline that is calculated using seekable.end. Without this fix, the SeekBar will still use the seekable.end of the previous source.
2. An issue where the SkipButton components are not rendered on the Sender when switching sources during a Chromecast session, due to the fact that buffered and seekable are not populated yet if the source change happened on the receiver.