Fixes https://github.com/spring-projects/spring-integration/issues/3266
It turns out that `AnnotationGatewayProxyFactoryBean.onInit()` implementation
parses a `@MessagingGateway` attributes ignoring possible properties
population by setters.
This way a Java DSL `GatewayProxySpec` becomes useless since all
its options are overridden by default values from a synthesized
`@MessagingGateway`.
Also it is inconsistency when we declare an `AnnotationGatewayProxyFactoryBean`
as regular bean, but then called setters are ignored
* Add `protected` getters into `GatewayProxyFactoryBean` for all
the properties which can be overridden by annotation attributes
* Fix `AnnotationGatewayProxyFactoryBean` to consult with those getters
before populating a property with value from the annotation
**Cherry-pick to 5.2.x**
dc15910f2c