SDK / exoplayer

- To make it clear that cache keys are for whole resources
- To explicitly make it clear to implementors that deriving a cache key
  from position and length is invalid. We've seen at least one developer
  trying to do this [1], so it seems worthwhile to be explicit!

[1] https://github.com/google/ExoPlayer/issues/5978#issuecomment-618977036

Issue: #5978
PiperOrigin-RevId: 312643930