SDK / exoplayer

The methods in ExoPlayerImpl and MediaControllerImplBase that determine
the new PlayerInfo/PlaybackInfo currently have a hard-to-reason-about
setup where the method generating the new info accesses other methods
that rely on the existing class field instead of working with the
passed in PlayerInfo/PlaybackInfo. This prevents reuse of the util
methods (e.g. for replaceMediaItems) because they access potentially
stale state.

This change untangles these methods a bit by making the util methods
either static or at least ensure that they don't rely on existing
class fields of PlayerInfo/PlaybackInfo. Overall, the change is a
complete no-op.

#minor-release

PiperOrigin-RevId: 534036633
(cherry picked from commit f898a7f17376d50c4de6cd8f67fe047410581527)