The flags is intended for the hotword stream from sound_trigger HAL.

When a stream has TRIGGER_ONLY flag set, CRAS will only invoke its callback once when data becomes available. And doesn't not send data to it anymore.

The purpose is to not have sound_trigger HAL receive any hotword data and let the subsequent hotword stream from audio HAL to receive hotword data.

This is needed because now that we transfer hotword stream in large chunks, if we allow sound_trigger HAL to read the samples, then a large portion of data have already been read. When audio HAL connects later, it cannot read the whole hotword data.