fix(android): validate auth callbacks before saving - #14888

Rejects malformed or mismatched custom URI authentication callbacks before changing stored authentication state, including callbacks for incomplete legacy requests without a nonce. Successful callbacks atomically store credentials and a hashed pending-handoff marker while consuming the nonce and CSRF state. Redelivery can recover an interrupted handoff until the callback activity starts the tunnel and replaces the authentication task with the main app. Acknowledgment then clears the marker so later replays cannot reconnect. New authentication requests clear the marker, while sign-out atomically clears credentials and any outstanding request. Concurrent duplicate delivery publishes only one terminal result.