This non-functional refactor inlines and simplifies the logic of
`BaseAudioProcessor` used by `SpeedChangingAudioProcessor`, and makes
the latter implement `AudioProcessor` directly.
`SpeedChangingAudioProcessor` acts as a wrapper over
`SonicAudioProcessor` and does not actually modify any samples,
so it had very little use for the affordances provided by
`BaseAudioProcessor`.
PiperOrigin-RevId: 698342962
66e8b53b43