Document AudioSystem::newAudioSessionId() failures

Change-Id: Iaa168722f362c36bdfa87fe20dc0a59b43cf1ca3
diff --git a/media/libmedia/IAudioFlinger.cpp b/media/libmedia/IAudioFlinger.cpp
index 762681e..f28b82d 100644
--- a/media/libmedia/IAudioFlinger.cpp
+++ b/media/libmedia/IAudioFlinger.cpp
@@ -599,7 +599,7 @@
         Parcel data, reply;
         data.writeInterfaceToken(IAudioFlinger::getInterfaceDescriptor());
         status_t status = remote()->transact(NEW_AUDIO_SESSION_ID, data, &reply);
-        int id = 0;
+        int id = AUDIO_SESSION_ALLOCATE;
         if (status == NO_ERROR) {
             id = reply.readInt32();
         }