先调用 ConsumerBase::setFrameAvailableListener() 设置 listener，然后给 mContentsChangedListener 赋值，它接收 onSidebandStreamChanged() 回调。

void BufferLayerConsumer::setContentsChangedListener(const wp<ContentsChangedListener>& listener) {
    setFrameAvailableListener(listener);
    Mutex::Autolock lock(mMutex);
    mContentsChangedListener = listener;
}
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struct ContentsChangedListener : public FrameAvailableListener {
    virtual void onSidebandStreamChanged() = 0;
};