Document AudioTrack mFrameCount and mReqFrameCount better

and remove unnecessary initialization of mFrameCount in set().

Change-Id: I9effeb0a6dd035ca02fe77f6992c55d9515b4df6
diff --git a/media/libmedia/AudioTrack.cpp b/media/libmedia/AudioTrack.cpp
index a6ffc62..7f6fd7a 100644
--- a/media/libmedia/AudioTrack.cpp
+++ b/media/libmedia/AudioTrack.cpp
@@ -318,7 +318,7 @@
     mVolume[LEFT] = 1.0f;
     mVolume[RIGHT] = 1.0f;
     mSendLevel = 0.0f;
-    mFrameCount = frameCount;
+    // mFrameCount is initialized in createTrack_l
     mReqFrameCount = frameCount;
     mNotificationFramesReq = notificationFrames;
     mNotificationFramesAct = 0;