[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>
diff --git a/services/audiopolicy/engineconfigurable/src/Engine.cpp b/services/audiopolicy/engineconfigurable/src/Engine.cpp
index 7e5c5e3..6d42fcf 100644
--- a/services/audiopolicy/engineconfigurable/src/Engine.cpp
+++ b/services/audiopolicy/engineconfigurable/src/Engine.cpp
@@ -129,7 +129,7 @@
     Element<Key> *element = getFromCollection<Key>(key);
     if (element == NULL) {
         ALOGE("%s: Element not found within collection", __FUNCTION__);
-        return BAD_VALUE;
+        return false;
     }
     return element->template set<Property>(property) == NO_ERROR;
 }