make DevServerHelper open (#51323)

Summary:
Expo inherits from the DevServerHelper class, and needs it to be declared as open, the same goes for its public interface.

Expo is using this in `DevLauncherDevServerHelper` and overrides the methods:

- getDevServerBundleURL
- getDevServerSplitBundleURL
- getSourceUrl
- getSourceMapUrl
- isPackagerRunning

This PR fixes this by adding the open to the class and to the methods that should be open

## Changelog:

[ANDROID] [FIXED] - Made DevServerHelper and its method open so that they can be overridden.

Pull Request resolved: https://github.com/facebook/react-native/pull/51323

Test Plan: Verify that we can build against Expo.

Reviewed By: cipolleschi

Differential Revision: D74876479

Pulled By: cortinico

fbshipit-source-id: 0037d6f7cee190a690ec3ec59896df04f46797b2