multimedia: Add API to set active output volume

Add cras utils and multimedia server API to set active output volume.

BUG=chromium:463813
TEST=not used yet.

Change-Id: I9557a8fe0f6b24b5c112508994543d4379b954c4
Reviewed-on: https://chromium-review.googlesource.com/257291
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
diff --git a/client/cros/audio/cras_utils.py b/client/cros/audio/cras_utils.py
index 33ca9b9..e53e11d 100644
--- a/client/cros/audio/cras_utils.py
+++ b/client/cros/audio/cras_utils.py
@@ -127,6 +127,16 @@
 
     return (output_match.group(1).strip(), input_match.group(1).strip())
 
+
+def set_selected_output_node_volume(volume):
+    """Sets the selected output node volume.
+
+    @param volume: the volume to be set (0-100).
+    """
+    selected_output_node_id, _ = get_selected_nodes()
+    set_node_volume(selected_output_node_id, volume)
+
+
 def get_active_stream_count():
     """Gets the number of active streams."""
     server_info = dump_server_info()