usb_audio_flow: Use audio_ as prefix for temporary files

Use audio_ as prefix like other flows so temporary files can be removed
when Reset is called.

BUG=chromium:601434
TEST=run audio_AudioBasicUSBPlayback test

Change-Id: If07cab41b6902cd5c433740e3a8822e026f5ee79
Reviewed-on: https://chromium-review.googlesource.com/337690
Commit-Ready: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
diff --git a/chameleond/utils/usb_audio_flow.py b/chameleond/utils/usb_audio_flow.py
index 5bd8447..a38169a 100644
--- a/chameleond/utils/usb_audio_flow.py
+++ b/chameleond/utils/usb_audio_flow.py
@@ -191,7 +191,7 @@
     self._supported_data_format.file_type = self._captured_file_type
     params_list = self._GetAlsaUtilCommandArgs(self._supported_data_format)
     file_suffix = '.' + self._captured_file_type
-    recorded_file = tempfile.NamedTemporaryFile(prefix='recorded',
+    recorded_file = tempfile.NamedTemporaryFile(prefix='audio_',
                                                 suffix=file_suffix,
                                                 delete=False)
     self._file_path = recorded_file.name