[AudioPolicy][EngineConfigurable] Fix new assistant stream regression

New assistant stream has been added.
This CL fixes the support of new stream within configurable engine.

Bug:
Test: build

Change-Id: If33aa6ea668655d933d274d9c46447185f75c8d0
Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>