CRAS: audio_thread - return error when last output thread is removed.
CRAS: audio_thread - return error when last output thread is removed.

Before an error would be returned when no streams were attached.
However this won't work going forward since a loopback stream could
remain.  Switch to checking if there are output streams instead of
checking if any streams are still attached.

BUG=chromiun:247160
TEST=unitests still pass and play/record audio works.

Change-Id: I893759e6a0a6adca59c375134776317c3a9ac8fa
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/60236
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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