CRAS: audio_thread - fix get min latency for loopback.
CRAS: audio_thread - fix get min latency for loopback.

The minimum latency loopback stream could be incorrect if a
lower-latency unified stream is present.  Fix that check.

BUG=chromium:304346
TEST=loopback audio with a unified stream playing.

Change-Id: I0884fea41430cea147b89d4fd0ad65722a577472
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172300

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