[Startup] "Initializing" spinner gets stuck on startup

This occurs when the app is opened in the background (e.g., via Android Studio) while the phone is locked, or after completing the Google Play Services install flow and returning to the app.

The root cause is that the sign in event occurs while the `MainActivity` isn't running. `MainViewModel` emits the `navigationRequests`, but `MainActivity` is not there to receive it. To resolve this:

* The `signInState` in `AuthenticationManager` should be a `StateFlow`. We have one now, but we convert to a `Flow` because we call `filterNotNull()`.
* Either Navigation events should only be triggered from within the Fragment lifecycle.