SDK / exoplayer

I moved this assignment in https://github.com/google/ExoPlayer/commit/326f53a61c4e58a9123561e0f0c838c40929852f
in order to provide a single source-of-truth for `publish.gradle`,
but as pointed out in Issue: androidx/media#416 this breaks apps that are depending
on our project locally using the instructions we publish. Instead we can
remove the `rootProject.name` check from `publish.gradle`, and check an
explicit boolean value instead to indicate if the root project is 'ours'
(with this boolean only set from `settings.gradle`, so it doesn't get
picked up by apps depending on us locally).

#minor-release

PiperOrigin-RevId: 534459085
(cherry picked from commit 3ca3e11de1ab7c8a1fde0f9dd92dd9724fedcc56)