Document AudioTrack mFrameCount and mReqFrameCount better

    float                   mVolume[2];

    float                   mSendLevel;

    mutable uint32_t        mSampleRate;            // mutable because getSampleRate() can update it.

    size_t                  mFrameCount;            // corresponds to current IAudioTrack

    size_t                  mReqFrameCount;         // frame count to request the next time a new

                                                    // IAudioTrack is needed

    size_t                  mFrameCount;            // corresponds to current IAudioTrack, value is

                                                    // reported back by AudioFlinger to the client

    size_t                  mReqFrameCount;         // frame count to request the first or next time

                                                    // a new IAudioTrack is needed, non-decreasing

    // constant after constructor or set()

    audio_format_t          mFormat;                // as requested by client, not forced to 16-bit

    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;