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An unique id per face while the face is visible to the tracker.
If the face leaves the field-of-view and comes back, it will get a new id.
This is an optional field, may not be supported on all devices. If the id is Face.IdUnsupported then the leftEyePosition, rightEyePosition, and mouthPositions are guaranteed to be null. Otherwise, each of leftEyePosition, rightEyePosition, and mouthPosition may be independently null or not-null. When devices report the value of key Android.Hardware.Camera2.CaptureResult.StatisticsFaceDetectMode as Android.Hardware.Camera2.CameraMetadata.STATISTICS_FACE_DETECT_MODE_SIMPLE in Android.Hardware.Camera2.CaptureResult, the face id of each face is expected to be Face.IdUnsupported.
This value will either be Face.IdUnsupported or otherwise greater than 0.