[JAVA] Remove [this-escape] warnings in generated ApiClient classes #960

Job	Run time
Build Java Client JDK8 (samples/client/echo_api/java/apache-httpclient)	1m 17s
Build Java Client JDK8 (samples/client/echo_api/java/resttemplate)	1m 14s
Build Java Client JDK8 (samples/client/echo_api/java/feign-gson)	59s
Build Java Client JDK8 (samples/client/echo_api/java/okhttp-gson)	1m 7s
Build Java Client JDK8 (samples/client/echo_api/java/resteasy)	51s
	5m 28s

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