[firebase_auth_ui][android] Add the SMS Retriever API

[nohe427]

Currently, when using one time passcodes with phone authentication, the user must visit their SMS messaging application and either memorize or select and copy the code to deliver back to their Flutter app. The SMS Retriever API, is an API that is designed to listen for a text message once an OTP password flow is created. Firebase already supports sending the message hash down to the application that is running and this would merely capitalize on the already existing efforts. This could reduce user friction by allowing the user to go through Google Play Integrity API (no reCaptcha flow supported here), and then once their app receives the message, automatically place it into the app and authenticate the user. The use of this API does not require any additional permissions and should be able to be added without too much friction.

Proposed Flow

1. User visits the Phone Auth flow
2. User enters their phone number
3. Play Integrity flow starts (automatically occurs from within the Android SDK)
4. Once a valid attestation flow completes, the user then is prompted to enter the OTP
5. The OTP is received from the SMS Retriever API and automatically delivered to the OTP screen within Firebase Auth UI and the number appears on the screen. The button to verify is then pressed automatically for the user advancing them to the next screen in their app.