Spring Move ApplicationContextAotInitializer to Spring Boot

AFAICT currently the only user of ApplicationContextAotInitializer is SpringApplication in Spring Boot. If we move the class to Spring Boot we can make it package-private and have more control.

Comment From: snicoll

I expect the TCF to use it as well. We also want to make sure custom frameworks can build a native image if they chose to. If anything, we should be moving more things to framework.