SDK / exoplayer

This makes it easier to reason about some parts of the code and
will eventually allow to easily switch between AudioProcessor-
based on AudioSink-based speed adjustment.

The current state saves the applicable playback parameters
in separate variables depending on which speed adjustment
path is used. Moreover, the AudioProcessor-based logic keeps
a chain of pending parameter changes and we derive the last
applicable one everytime we need the current parameters.

After this change, this is simplified by
 - keeping a common value for playback parameters independent
   of the actual path we use for adjustment.
 - keeping the final ("current") parameters directly, instead
   of deriving it from a chain of yet to be applied parameters.

PiperOrigin-RevId: 505097294