fix(firebase_ui_auth): Use widget.email in ForgotPasswordScreen

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?

Checklist

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[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
[x] All existing and new tests are passing.
[x] I updated/added relevant documentation (doc comments with ///).
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[x] All unit tests pass (melos run test:unit:all doesn't fail).
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Breaking Change

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[ ] Yes, this is a breaking change.
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firebase / FirebaseUI-Flutter