Ensure that Amazon Voice Focus only receives downmixed mono input.

[richnew10]

When the SDK requests an audio input, it specifies ideal
single-channel input. The browser is not guaranteed to return a mono
stream, and indeed builders can supply a stereo stream from their own
audio sources.

This change ensures that the Web Audio graph will provide a
single-channel mixed input to the node, and thus the processor will
receive only a single array of samples.

1. Have you successfully run npm run build:release locally?

Yes.

2. How did you test these changes?

In the Voice Focus test harness, and manually in the demo app.

3. Can these changes be tested using the demo application? If yes, which demo application can be used to test it?

Not directly. You can fake it:

const gUM = navigator.mediaDevices.getUserMedia;
navigator.mediaDevices.getUserMedia = (constraints) => {
  const c = constraints.audio ? {...constraints, audio: {...constraints.audio, channelCount: 2 } } : constraints;
  console.info('XXX using constraints', c);
  return gUM.call(navigator.mediaDevices, c);
};

which will cause stereo inputs to be returned. If you hit the Record button you'll get a recording that now plays through both channels, which was not previously the case.

4. Do you add, modify, or delete public API definitions? If yes, has that been reviewed and approved?

No.

5. Do you change the wire protocol, e.g. the request method? If yes, has that been reviewed and approved?

No.

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