Refactor ReactDelegate to provide DevSupportManager (#43520)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43520

Refactor ReactDelegate to have a private `getDevSupportManager()` that can also be re-used  by `reload()`

This method conditionally provides the correct DevSupportManager in cases of Bridge & Bridgeless

Changelog:
[Internal] internal

Reviewed By: cortinico

Differential Revision: D54967130

fbshipit-source-id: 37d585de33a50b98d01803d3080c5693a8c494b9