CRAS: audio_thread - pad unified output on opening.
CRAS: audio_thread - pad unified output on opening.

Having this in the normal playback path has caused issues.  Pad it once
when opening.  This is particularly safe right now since Chrome only
used this path for a couple releases and currently the unified path sits
idle on ChromeOS.

BUG=none
TEST=play unified stream using test_client.

Change-Id: I2fed786ae45b4b546e6d4b2a61a368252363594b
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172939
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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