audio: support multiple mixer values per control

Multiple mixer control values can now be set by using
the optional id xml attribute. For example:

<ctl name="Volume" id="0" value="13">
<ctl name="Volume" id="1" value="15">

Would set the channel at index 0 (usually left) to
13, and the other channel to 15.

Change-Id: Id135d5a8175b3d89b0f2be69e9b1c014d95da2f2
1 file changed