fix(firebase_ui_auth): Use widget.email in ForgotPasswordScreen

[kzrnm]

Description

When EmailForm calls showForgotPasswordScreen, the email address entered in EmailInput is used as the default value.
However, if EmailForm has email as field, an empty string will be used as the default value.

I have not written a test yet because of the difficulty in creating a mock showForgotPasswordScreen. Is there a good way to do this?

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[russellwheatley]

Thanks for the PR! I made some modifications and it has already been merged: #336