Fix spotbugs bugs

[zoewangg]

• Enabled findbugs <failOnError>true</failOnError> and fixed reported bugs
• Clean up code a bit

[varunnvs92]

Are we sure all the new values are false positives?
For example, this code seems to be unsafe

[zoewangg]

Yes, this is false positive.

We are passing SdkHttpFullResponse to headersReceived method.

aws-sdk-java-v2/http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/ResponseHandler.java

Lines 68 to 74 in bffab64

	SdkHttpResponse sdkResponse = SdkHttpFullResponse.builder()
	.headers(fromNettyHeaders(response.headers()))
	.statusCode(response.status().code())
	.statusText(response.status().reasonPhrase())
	.build();
	channelContext.channel().attr(KEEP_ALIVE).set(HttpUtil.isKeepAlive(response));
	requestContext.handler().headersReceived(sdkResponse);