FR: Migrate to shared Keychain without clearing signed in user

### Feature proposal

* Firebase Component: Auth

Currently migrating to a shared Keychain means clearing the current user. For an app in production with thousands of MAU, signing out all of those users to migrate their keychain.

Would it be possible to move the login tokens from a non-shared keychain to a shared keychain, as it seems possible to move in the other direction.

The issue identifies Firebase Auth and migration from a non-shared to a shared Keychain; begin by tracing the existing keychain migration path and its handling of the signed-in user. Done means users retain their login state while tokens move to the shared Keychain, without clearing the current user.

Written by the indexing model from the issue text.