Expose a public getter on a TracingAgent for background mode (#54282)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/54282

# Changelog: [Internal]

It is already in a tracing session state, we are going to use it in RuntimeTarget.

Reviewed By: huntie

Differential Revision: D85440638

fbshipit-source-id: 34e9a485b2701580f6ceef4262537a5a428dfcee