Pass stereo gains as packed minifloat

This will allow (eventually) a greater dynamic range for gains.
However there are still a few remaining places in effects and mixer
that will also need to be changed in order to get the full benefit.

Also fixes a minor bug: was not checking for NaN in AudioTrack C++.

Change-Id: I63bce9e82e0a61546d8ff475fb94bcb700d99c96
diff --git a/services/audioflinger/PlaybackTracks.h b/services/audioflinger/PlaybackTracks.h
index 08b1728..6f1f293 100644
--- a/services/audioflinger/PlaybackTracks.h
+++ b/services/audioflinger/PlaybackTracks.h
@@ -65,7 +65,7 @@
             void        signal();
 
 // implement FastMixerState::VolumeProvider interface
-    virtual uint32_t    getVolumeLR();
+    virtual gain_minifloat_packed_t getVolumeLR();
 
     virtual status_t    setSyncEvent(const sp<SyncEvent>& event);