[0.80] Make accessors inside HeadlessJsTaskService open again

[naquilini]

Target Branch

0.80

Link to commit or PR to be picked

facebook/react-native#52660

Description

The documentation in HeadlessJsTaskService for getReactHost and getReactNativeHost methods mention that users should override them to provide their own implementation.
However those vals are not open so users cannot really override them.

[react-native-bot]

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