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Similar to ALSA snd_pcm_get_poll_fd, this will allow a user to wait for
a sound device to have data in a poll(2) call.  The hotword thread will
use this to wait for a message on its socket or for audio data to become
ready.

Change-Id: I61375a73d7a842b00362fc707c12f99aa18e0bdf
Signed-off-by: Dylan Reid <dgreid@chromium.org>