[WIP) [media] Support sync MS init segment append

[xiaomings]

b/271167555

[github-actions]

🤖 Gemini Suggested Commit Message

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media: Support synchronous MSE init segment append

Introduce a new H5vcc setting, Media.AppendFirstSegmentSynchronously,
to allow the first media segment appended to a SourceBuffer to be
processed synchronously.

Performing the initial segment parsing and appending synchronously
reduces delays associated with asynchronous processing. This improves
media startup times for Cobalt applications, especially on platforms
that utilize the Starboard media backend.

Bug: b/271167555

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