SDK / exoplayer

I think this has been broken since https://github.com/google/ExoPlayer/commit/617267bfcf20c500fb4a3cfdff7104e4d88261a7 (which was trying to fix
the same problem).

This change initializes `rootDir` to always be the current project (i.e. ExoPlayer)
directory. From the [Gradle docs](https://docs.gradle.org/current/userguide/working_with_files.html#sec:single_file_paths):
> What happens in the case of multi-project builds? The file() method
> will always turn relative paths into paths that are relative to the
> current project directory, which may be a child project.

We can also then remove exoplayerRoot completely and simplify the local
dependency instructions.

* #minor-release
* #exofixit
* Issue: #9403

PiperOrigin-RevId: 395478121