Communicate audio session ID to downmixer

The audio downmixer effect might need the audio session Id, pass it
 from the track creation in AudioFlinger to the downmix effect
 creation in AudioMixer.

Change-Id: I836873eebd6711f1048fce81cd2eb29b94f0ad0c
diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index 6c80253..dc468ff 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -91,7 +91,7 @@
     // For all APIs with "name": TRACK0 <= name < TRACK0 + MAX_NUM_TRACKS
 
     // Allocate a track name.  Returns new track name if successful, -1 on failure.
-    int         getTrackName(audio_channel_mask_t channelMask);
+    int         getTrackName(audio_channel_mask_t channelMask, int sessionId);
 
     // Free an allocated track by name
     void        deleteTrackName(int name);
@@ -190,7 +190,9 @@
 
         DownmixerBufferProvider* downmixerBufferProvider; // 4 bytes
 
-        int32_t     padding[3];
+        int32_t     sessionId;
+
+        int32_t     padding[2];
 
         // 16-byte boundary