Fix popping sound when silencing audio

[taquitos]

After selecting a sensor:
Enable Audio, notice that it plays back the current value as a tone.
Now, disable the audio playback. There is a distinct popping sound. Can we get rid of that?

Might start here:
https://github.com/google/science-journal-ios/blob/be7ecf0c3749793d6b18c1058d343a66c6d4314d/ScienceJournal/ToneGenerator/ToneGenerator.swift#L79