SDK / exoplayer

- WavHeader is now immutable and contains only values parsed out
  of the WAVE FMT chunk. It no longer contains a C.PcmEncoding
  encoding, or mutable data bounds.
- WavHeaderReader now parses the WAVE header chunks without any
  additional logic (e.g. validating the block alignment value,
  which is format type dependent).
- The SeekMap part of WavHeader is split out into WavSeekMap.

PiperOrigin-RevId: 285232498