When flushing the AudioGraph, the AudioMixer will be preserved but the
sources will be recreated. This is probably a bit less efficient but
makes the logic simpler. Indeed, if we had to keep the sources alive,
we would need to add a way to reconfigure them with a new timestamp for
seeking. We would also need to change the way sources are ended because
they are currently removed when they are ended. Also, it is acceptable
to have a small delay when seeking, which means that performance is less
critical than for playback.
PiperOrigin-RevId: 615775501
e40ce150bf