Previously, ConfigFileApplicationListener was listed in spring.factories
as both an EnvironmentPostProcessor and an ApplicationListener. This
was problematic as ConfigFileApplicationListener is stateful and listing
it twice lead to two separate instances with separate state.

This commit restore ConfigFileApplicationListener to only being an
ApplicationListener. The driving of EnvironmentPostProcessors that was
performed by EnvironmentPostProcessingApplicationListener is now
performed by ConfigFileApplicationListener which adds itself as an
EnvironmentPostProcessor. This ensures that there’s only a single
instance of ConfigFileApplicationListener, allowing its state to be
managed correctly.

Closes gh-4258