Use symbol AUDIO_DEVICE_NONE from <system/audio.h>

Change-Id: I61f882c5e7c949bf00d3bfc745ebf3b5e1c42a58
diff --git a/media/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index 140fb66..dafccd6 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -739,7 +739,7 @@
 audio_devices_t AudioSystem::getDevicesForStream(audio_stream_type_t stream)
 {
     const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
-    if (aps == 0) return (audio_devices_t)0;
+    if (aps == 0) return AUDIO_DEVICE_NONE;
     return aps->getDevicesForStream(stream);
 }