[BUG] audiopolicy: engineconfigurable: prevent illegal map access

If any request to get OutputDevice for a given attributes is performed
before the cache has been initalized, an illegal access to the vector
happens.
This CL fixes by returning empty device vector when cache is empty.

Test: build & boot configurable engine target & basic audio

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: I1371765401441732319be56a9cae6fbf3c5cc2bf
diff --git a/services/audiopolicy/engineconfigurable/src/Engine.h b/services/audiopolicy/engineconfigurable/src/Engine.h
index d8e2742..f665da5 100644
--- a/services/audiopolicy/engineconfigurable/src/Engine.h
+++ b/services/audiopolicy/engineconfigurable/src/Engine.h
@@ -126,6 +126,7 @@
     status_t loadAudioPolicyEngineConfig();
 
     DeviceVector getDevicesForProductStrategy(product_strategy_t strategy) const;
+    DeviceVector getCachedDevices(product_strategy_t ps) const;
 
     /**
      * Policy Parameter Manager hidden through a wrapper.