Use audio_io_handle_t consistently instead of int

Other:
 - add a comment to nextUniqueId
 - made ThreadBase::mId const, since it is only assigned in constructor.

Change-Id: I4e8b7bec4e45badcde6274d574b8a9aabd046837
diff --git a/include/media/AudioSystem.h b/include/media/AudioSystem.h
index 74a1e62..da99620 100644
--- a/include/media/AudioSystem.h
+++ b/include/media/AudioSystem.h
@@ -55,8 +55,10 @@
     static status_t getMasterMute(bool* mute);
 
     // set/get stream volume on specified output
-    static status_t setStreamVolume(audio_stream_type_t stream, float value, int output);
-    static status_t getStreamVolume(audio_stream_type_t stream, float* volume, int output);
+    static status_t setStreamVolume(audio_stream_type_t stream, float value,
+                                    audio_io_handle_t output);
+    static status_t getStreamVolume(audio_stream_type_t stream, float* volume,
+                                    audio_io_handle_t output);
 
     // mute/unmute stream
     static status_t setStreamMute(audio_stream_type_t stream, bool mute);
@@ -217,7 +219,7 @@
 
         // indicate a change in the configuration of an output or input: keeps the cached
         // values for output/input parameters upto date in client process
-        virtual void ioConfigChanged(int event, int ioHandle, void *param2);
+        virtual void ioConfigChanged(int event, audio_io_handle_t ioHandle, void *param2);
     };
 
     class AudioPolicyServiceClient: public IBinder::DeathRecipient