Rename const variable to SIGN_IN_RESULT_CODE

[secretshardul]

The starter and finished code use a const variable SIGN_IN_RESULT_CODE. However this variable is incorrectly named as SIGN_IN_REQUEST_CODE in comments.

The Codelabs tutorial task 6: Implement the login button also used the name SIGN_IN_REQUEST_CODE. This should also be changed by maintainers.

Fixes #5 and #9

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[secretshardul]

@googlebot I signed it!

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