Clean up demo app samples
Clean up demo app samples

- Retire YouTube streams. They're flaky (e.g., one of them has transformed into
  an audio only stream!). There is also a desire for us to stop relying on special
  non-expiring YouTube URLs.
- Reorganize the remaining streams.

PiperOrigin-RevId: 344421031

androidx/media,amzn/exoplayer-amazon-port,google/ExoPlayer,amzn/exoplayer-amazon-port,google/ExoPlayer,ened/ExoPlayer,google/ExoPlayer,amzn/exoplayer-amazon-port,androidx/media,ened/ExoPlayer,ened/ExoPlayer,androidx/media