Adjust frag_duration setting in stream

[uvjustin]

Proposed change

As I was doing more testing with LL-HLS I noticed a playback artifact when using hls.js with one of my cameras. The playback loader is consistently skipping the last part in a segment. This can be addressed somewhat in the frontend via playback options, but I think the cause is when the media fragments are consistently slightly too long and the metadata reported is slightly shorter. Near the end of the segment, the loader sees that the duration of the media it has already equals the total duration indicated in the metadata, so the last part is skipped. However, when actually concatenating the media itself, there is now a hole where the missing fragment should belong, which can cause playback problems.
Previously the fragmenting strategy was to use a higher frag_duration parameter when streams have audio to avoid having any parts which are too short, as having any of these seems to break the iOS native player. However, with the addition of a floor for part duration metadata in #58036, this may not be a problem as long as the average part duration is still ok. Assuming this is the case, risking a too short part duration is better than risking a too long part duration, as a too long part duration may cause more issues with the last part having an inaccurate duration.
This value seems to work better for my cameras, but we may need to tweak things further or perhaps make this frag_duration multiplier a tunable parameter depending on what other users experience.

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