Change parameter type for volume to float in AudioMixer

Change-Id: I4da1505ce852505f86f8e5b87f60e8edceeb30e0
diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index 43fe664..766ff60 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -59,6 +59,7 @@
     static const uint32_t MAX_NUM_CHANNELS_TO_DOWNMIX = 8;
 
     static const uint16_t UNITY_GAIN_INT = 0x1000;
+    static const float    UNITY_GAIN_FLOAT = 1.0f;
 
     enum { // names