[google_sign_in] Add GoogleSignInExceptionCode export (#9545)
GoogleSignInException and GoogleSignInExceptionCode were both intended to be exported to clients to allow for structured error handling, but the latter was missed. This adds the missing export, as well as example usage in the example app (which would have caught the mistake).
Fixes https://github.com/flutter/flutter/issues/171462
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[google_sign_in] Use an activity for credential requests (#9497)
getCredentialManager is documented to require an activity context, not an application context; this fixes it to use the activity context since in some configurations using the application context will throw an error.
Fixes https://github.com/flutter/flutter/issues/171122
## Pre-Review Checklist
[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.