HarmonyOS base64的图片如何转成Arraybuffer

使用@ohos.util (util工具函数)模块的Base64Helper进行base64解码，获取Uint8Array对象，然后获取ArrayBuffer。

https://developer.huawei.com/consumer/cn/doc/harmonyos-references-V5/js-apis-util-V5#decodesync9

参考代码

let that = new util.Base64Helper();
let buff = 'TWFuaXNkaXN0aW5ndWlzaGVkb******************************************Nz\r\naW9uZnJvbW90aGVyYW5pbWFsc3doaWNoaXNhbHVzdG9mdGhlbWluZGV4Y2VlZHN0aGVzaG9ydHZl\r\naGVtZW5jZW9mYW55Y2FybmFscGxlYXN1cmU=\r\n';
let result:Uint8Array = that.decodeSync(buff, util.Type.MIME);
let buf:ArrayBuffer = result.buffer as ArrayBuffer
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