Add prerelease staging API for Android (#49649)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49649

- Added a new template to React Native's Feature Flag's script for Canary and Experimental prerelease stages
- Added a parameter to `DefaultNewArchitectureEntryPoint.kt` to select prerelease stage

Changelog: [Android] [Added] - On `DefaultNewArchitectureEntryPoint` class add property to specify the desired release level for an application

Reviewed By: rubennorte, mdvacca

Differential Revision: D69412971

fbshipit-source-id: 1a76ac723e1e06b40aad5910604e0384b208d3a5