SDK / exoplayer

There are two overloads of this method due to a type 'rename' from
`PlayerControlView.VisibilityListener` to
`PlayerView.ControllerVisibilityListener`. Currently when you call one
overload it passes `null` to the other one (to clear the other listener).
Unfortunately this results in it clearing itself, because it receives
a null call back!

This change tweaks the documentation to clarify that the 'other'
listener is only cleared if you pass a non-null listener in. This solves
the recursive problem, and allows the 'legacy' visibility listener to be
successfully registered.

Issue: androidx/media#229

#minor-release

PiperOrigin-RevId: 496876397
(cherry picked from commit 37fd65a8)