This more closely matches the pattern we have for all implementations
except DefaultMediaSourceFactory (e.g. ProgressiveMediaSource.Factory)
and other factory interfaces like (Http)DataSource.Factory.
PiperOrigin-RevId: 417826803
This mime type is technically for the Muxer, and determines
the container used. In the context of the transformer, this can
be thought of more as a container mime type, to avoid confusion
with the video mime type and audio mime type.
Deprecates setOutputMimeType().
PiperOrigin-RevId: 410530707