ReleaseBufferCallback TransactionCompletedListener::popReleaseBufferCallbackLocked(
        const ReleaseCallbackId& callbackId) {
    ReleaseBufferCallback callback;
    auto itr = mReleaseBufferCallbacks.find(callbackId);
    if (itr == mReleaseBufferCallbacks.end()) {
        return nullptr;
    }
    callback = itr->second;
    mReleaseBufferCallbacks.erase(itr);
    return callback;
}

• 从 mReleaseBufferCallbacks 查找 callbackId 对应的 ReleaseCallbackId。
• 从 mReleaseBufferCallbacks 移除该元素并返回。
• 在 Transaction::setBuffer() 中加入到 mReleaseBufferCallbacks。