* Check if reverse client ID is registered as a custom URL scheme before setting it as the callback scheme.
* Fix existing tests.
* Add tests.
* Update changelog.
* Use GOOGLE_APP_ID instead of CLIENT_ID if CLIENT_ID is unavailable for generic IDP and phone auth flows.
* Add a prefix to the custom URL scheme when using the Firebase app ID.
* Add tests for FIROAuthProvider.
* Add tests for FIRPhoneAuthProvider.
* Change query param from firebaseAppId to appId.
* Fix tests.
* Address review comments.
* Add changelog entry.