[google_sign_in] Add Android account name field as optional (platform interface changes)

[neilself]

This PR contains the platform interface changes for the main PR 8573. As a whole, these changes add a new option to explicitly specify the account name on Android while signing in within the google_sign_in package.

Flutter issue: 163257

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[stuartmorgan]

LGTM

@camsim99 for second approval