Can not specify an audio effect for the default stream

The default stream is an alias to an other stream.
One can not allocate effects for it in the configuration
as it could conflict with the stream it aliases.

Test: run xmllint against all audio effect configs
Bug: 37492580
Change-Id: Ia2bbc13b5cf2a0a40e5eeff7ee22a7423ec16767
Signed-off-by: Kevin Rocard <krocard@google.com>