To reliably reject the System UI playback resumption notification on
all API levels (specifically API 30), the backward compatibility layer
needs to return `null` for the library root.
This is not possible in the Media3 implementation. This change allows
an app to return a `LibraryResult.ofError(RESULT_ERROR_NOT_SUPPORTED)`
that then is translated to return null by the backwards compatibility
layer.
Issue: androidx/media#355
Issue: androidx/media#167
Issue: androidx/media#27
See https://developer.android.com/guide/topics/media/media-controls#mediabrowserservice_implementation
PiperOrigin-RevId: 527276529
7938978b51