When clipping a MediaItem with start time > 0, the audio was ending
before the video. This is because:
- Audio timestamps are computed based on the sample sizes, with a start
time set to streamOffsetUs (i.e. the streamStartPositionUs is not
taken into account).
- The SamplePipeline was subtracting streamStartPositionUs from the
timestamps before sending the samples to the muxer.
- As a result, the audio timestamps were shifted by
streamStartPositionUs, while they should be shifter by streamOffsetUs.
PiperOrigin-RevId: 511175923
