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/services/audioflinger/AudioPolicyService.h b/services/audioflinger/AudioPolicyService.h
index 62219e5..fdaf576 100644
--- a/services/audioflinger/AudioPolicyService.h
+++ b/services/audioflinger/AudioPolicyService.h
@@ -174,8 +174,10 @@
                     void        startToneCommand(ToneGenerator::tone_type type,
                                                  audio_stream_type_t stream);
                     void        stopToneCommand();
-                    status_t    volumeCommand(audio_stream_type_t stream, float volume, int output, int delayMs = 0);
-                    status_t    parametersCommand(int ioHandle, const char *keyValuePairs, int delayMs = 0);
+                    status_t    volumeCommand(audio_stream_type_t stream, float volume,
+                                            audio_io_handle_t output, int delayMs = 0);
+                    status_t    parametersCommand(audio_io_handle_t ioHandle,
+                                            const char *keyValuePairs, int delayMs = 0);
                     status_t    voiceVolumeCommand(float volume, int delayMs = 0);
                     void        insertCommand_l(AudioCommand *command, int delayMs = 0);
 
@@ -207,12 +209,12 @@
         public:
             audio_stream_type_t mStream;
             float mVolume;
-            int mIO;
+            audio_io_handle_t mIO;
         };
 
         class ParametersData {
         public:
-            int mIO;
+            audio_io_handle_t mIO;
             String8 mKeyValuePairs;
         };