CRAS: Move append_stream from audio_thread into dev_io

Moving append_stream into dev_io to make logic cleaner. It can provide
better test surface for unittest.

BUG=chromium:932381
TEST=Audio works normally on peppy.

Change-Id: Iec9bd79c08d11c041036262db899ac9a23c9bd87
Reviewed-on: https://chromium-review.googlesource.com/1584200
Commit-Ready: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Tested-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
5 files changed