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/media/libmedia/AudioTrackShared.cpp b/media/libmedia/AudioTrackShared.cpp
index 323b675..27a3718 100644
--- a/media/libmedia/AudioTrackShared.cpp
+++ b/media/libmedia/AudioTrackShared.cpp
@@ -27,7 +27,7 @@
 
 audio_track_cblk_t::audio_track_cblk_t()
     : mServer(0), mFutex(0), mMinimum(0),
-    mVolumeLR(0x10001000), mSampleRate(0), mSendLevel(0), mFlags(0)
+    mVolumeLR(GAIN_MINIFLOAT_PACKED_UNITY), mSampleRate(0), mSendLevel(0), mFlags(0)
 {
     memset(&u, 0, sizeof(u));
 }