Refactor/ bugfix for ReactModalHostView (#46326)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46326

- renaming variabled to make intent more clear
    - `dialog` -> `dialogWindow` to distinguish with `activity.window`
    - `hostView` -> `dialogRootViewGroup` as name was confusing.
        - `ReactModalHostView` creates and manages `DialogRootViewGroup` but it used as contentView for the Dialog.
- bug fixes

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D62177564

fbshipit-source-id: f81b167c1a234c02617ec2a3d63979628e01063b