AWSMobileClient auth = AWSMobileClient.getInstance();
auth.signIn("username", "password", new HashMap<String, String>());
if (auth.isSignedIn()) {
        ActivityUtils.startActivity(this, MyActivity.class);
}

Role ARN    arn:aws:iam::<my account number>:role/Cognito_S3_Full_Access

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "s3:*",
            "Resource": "*"
        }
    ]
}

AWSMobileClient awsMobileClient =AWSMobileClient.getInstance();
AmazonS3Client s3Client = new AmazonS3Client(awsMobileClient);

2019-05-20 17:56:21.645 27019-27019/com.mycomp.mypackage E/S3Uploader: Error upload file to S3 bucket : Access Denied (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: 375C07384A904483)

BasicAWSCredentials credentials = new BasicAWSCredentials("myAccessKey_hsjgfhjsdg", "mySecretKey_asdhasjkdha");
AmazonS3Client s3Client = new AmazonS3Client(credentials);