Make accessors inside HeadlessJsTaskService open again (#52660)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52660

The documentation for those methods mention that users should override them to
provide their own implementation.

However those vals are not `open` so users cannot really override them.
This fixes it.

See more context on https://github.com/facebook/react-native/pull/48800#issuecomment-3082665024

So this was practically a breaking change, that I'm attempting to mitigate.

Changelog:
[Android] [Fixed] - Make accessors inside HeadlessJsTaskService open again

Reviewed By: cipolleschi

Differential Revision: D78479162

fbshipit-source-id: eefc7332e2004198cd6bd64b60a66215f137ad4a