KmsMasterKeyProvider regionalClientSupplier_ does not appear to be getting set correctly by default

When a `KmsMasterKeyProvider` is constructed using the default builder[1] and is then used to attempt to decrypt a ciphertext message from another region, the decrypt call fails with this error[2].

It appears that something is not connecting with the default supplier in `build()`[3].

[1] `KmsMasterKeyProvider.builder().build()`
[2] https://github.com/aws/aws-encryption-sdk-java/blob/6cb1c4256e26f26ecfd557608b80c9ed2bba4141/src/main/java/com/amazonaws/encryptionsdk/kms/KmsMasterKeyProvider.java#L526
[3] https://github.com/aws/aws-encryption-sdk-java/blob/6cb1c4256e26f26ecfd557608b80c9ed2bba4141/src/main/java/com/amazonaws/encryptionsdk/kms/KmsMasterKeyProvider.java#L245-L259

从 src/main/java/com/amazonaws/encryptionsdk/kms/KmsMasterKeyProvider.java 开始，阅读第 245-259 行的默认 builder() 路径，以及第 526 行附近的解密失败位置。使用 KmsMasterKeyProvider.builder().build() 重现构造过程，并解密来自其他区域的 ciphertext；当默认的 regionalClientSupplier 正确连接且跨区域解密不再失败时，即表示完成。

由索引模型根据 Issue 内容生成。