Let state machine handle password changes from web channel

[skhamis]

When I was adding password change notification in Fenix via: https://phabricator.services.mozilla.com/D301414

I noticed in app-services we were potentially dropping push events on transient errors. The old version used ? on the introspect call, so I think any network/server issues propagated as Err out of handle_push_message. The Kotlin side's handleFxaExceptions would log it and ignore. So no event reached the FSM, no UI change leading to -> https://bugzilla.mozilla.org/show_bug.cgi?id=1915765

So we should lean into the state machine and let it be the source of truth imo. The state machine can decide whether the token is valid/retry/etc instead of push.

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[mhammond]

this lgtm, thanks - am a little concerned about iOS now re-registering twice though?

[mhammond]

forgot to say: shouldn't "handle login" also be an event in this world?