Duration was originally included in MediaFormat to match the
framework class, but it actually doesn't make much sense. In
many containers there's no such thing as per-stream duration,
and in any case we don't really care. Setting the duration on
each format required excessive piping.
This change moves duration into SeekMap instead, which seems
to make a lot more sense because it's at the container level,
and because being able to seek is generally couplied with
knowing how long the stream is.
This change is also a step toward merging Format and MediaFormat
into a single class (because Format doesn't have a duration),
which is coming soon.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=114428500
