[autotest] audio_Aplay: Change function of selecting output node

There is a bug in set_selected_output_nodes function. So use
set_single_selected_output_node instead.

BUG=chromium:816823
TEST=run audio_Aplay and audio_Aplay.headphone test on peppy and teemo.

Change-Id: Ieb707d633800a6b2bf0b2b2ace20775112871b37
Reviewed-on: https://chromium-review.googlesource.com/1300837
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
diff --git a/client/site_tests/audio_Aplay/audio_Aplay.py b/client/site_tests/audio_Aplay/audio_Aplay.py
index 48df5a5..2048707 100644
--- a/client/site_tests/audio_Aplay/audio_Aplay.py
+++ b/client/site_tests/audio_Aplay/audio_Aplay.py
@@ -112,7 +112,7 @@
                 logging.debug("No internal speaker. Skipping the test.")
                 return
 
-        cras_utils.set_selected_output_nodes([output_node])
+        cras_utils.set_single_selected_output_node(output_node)
 
         cras_device_type = cras_utils.get_selected_output_device_type()
         logging.debug("Selected output device type=%s", cras_device_type)