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/AudioMixer.h b/services/audioflinger/AudioMixer.h
index e5e120c..09e63a6 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -30,6 +30,9 @@
 #include <system/audio.h>
 #include <media/nbaio/NBLog.h>
 
+// FIXME This is actually unity gain, which might not be max in future, expressed in U.12
+#define MAX_GAIN_INT AudioMixer::UNITY_GAIN
+
 namespace android {
 
 // ----------------------------------------------------------------------------
@@ -91,6 +94,7 @@
         REMOVE          = 0x4102, // Remove the sample rate converter on this track name;
                                   // the track is restored to the mix sample rate.
         // for target RAMP_VOLUME and VOLUME (8 channels max)
+        // FIXME use float for these 3 to improve the dynamic range
         VOLUME0         = 0x4200,
         VOLUME1         = 0x4201,
         AUXLEVEL        = 0x4210,