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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: Ia9f5ffc67d458cd9466dc4f090a5be85d9738d1f
Signed-off-by: Dylan Reid <dgreid@chromium.org>