[firebase_auth_ui][android] Add the Phone Number Hint API

[nohe427]

Currently when a user is signing up for an app, they must select their country code and their mobile number. It may be worthwhile to add the Phone Number Hint API, which does not require additional permissions, and can allow the user to select their phone number from a list that is presented on the screen. This could then populate the phone number screen when the user wants to use phone authentication, saving themselves time from typing in their phone number and selecting their country code.

Proposed Flow

1. Developer enables the Phone Number Hint API as an optional flag (or potentially an action)
2. User visits the Phone Auth flow
3. If the action or flag is activated, the phone number hint pending intent is launched.
   a. User selects their phone number
   i. Number is pre-populated in the box and the user can select to continue
   b. User closes the box
   i. Number is not pre-populated in the box and the user can choose to enter their phone number to continue