The immediate plan is to replace `Transformer Muxer interface`
with this one.
It was not straight forward to move/change `Transformer Muxer interface`
hence this intermediate step.
The follow up CL will also remove fragmented MP4 related code from
`Mp4Muxer` and move it into `FragmentedMp4Muxer implements Muxer`.
PiperOrigin-RevId: 618789265
483d2bb9e7