`PreloadMediaSource` allows to have a `startPositionUs` passed when `preload` is called, then in `PreloadControl.onContinueLoadingRequested`, it can be more intuitive to see the buffered duration rather than the absolute buffered position as the preload progress. Similar in `DefaultPreloadManager`, we haven't allowed the apps to set a custom start position for individual sources though, once we add this support, using the "duration from the start position" than the absolute position will be less error-prone, otherwise, it can run into a case that the position that the apps set is smaller than the start position.
PiperOrigin-RevId: 674251362
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