These aren't caught by presubmit because the classes are annotated
`@VisibleForTesting` and are therefore stripped out by Metalava. However
Metalava doesn't run when we're generating javadoc for real.
#minor-release
PiperOrigin-RevId: 487476260
ExoPlayer extensions are modules that depend on external libraries to provide
additional functionality. Browse the individual extensions and their READMEs to
learn more.