audio policy: use audio_devices_t when appropriate

Change-Id: I1b3a5879e81c789fb53d356af3d3a1ee2dca955f
diff --git a/media/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index ec4c044..404853c 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -701,10 +701,10 @@
     return aps->getStrategyForStream(stream);
 }
 
-uint32_t AudioSystem::getDevicesForStream(audio_stream_type_t stream)
+audio_devices_t AudioSystem::getDevicesForStream(audio_stream_type_t stream)
 {
     const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
-    if (aps == 0) return 0;
+    if (aps == 0) return (audio_devices_t)0;
     return aps->getDevicesForStream(stream);
 }