audio policy: use audio_devices_t when appropriate

Change-Id: I1b3a5879e81c789fb53d356af3d3a1ee2dca955f
diff --git a/include/media/AudioSystem.h b/include/media/AudioSystem.h
index 89bc95d..e20f1fd 100644
--- a/include/media/AudioSystem.h
+++ b/include/media/AudioSystem.h
@@ -185,7 +185,7 @@
                                          audio_devices_t device);
 
     static uint32_t getStrategyForStream(audio_stream_type_t stream);
-    static uint32_t getDevicesForStream(audio_stream_type_t stream);
+    static audio_devices_t getDevicesForStream(audio_stream_type_t stream);
 
     static audio_io_handle_t getOutputForEffect(effect_descriptor_t *desc);
     static status_t registerEffect(effect_descriptor_t *desc,