[google_sign_in] Simply Kotlin/Java interop utils

[stuartmorgan-g]

Replaces one-off wrappers with a templated version. While this isn't important in isolation, it sets a better precedent for us to follow for future plugins, and likely for Pigeon generation of helper utils.

The "Unit" forms of the wrappers are still necessary because there's no way (AFAICT) to cause Java to infer the Unit type, since it's not a thing in Java.

I've validated that the type inference on the generic is still enforcing type safety based on the inferred type. I.e., if I try to pass the wrong value type, it won't compile.

Related to flutter/flutter#158287

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[gemini-code-assist]

Code Review

This pull request introduces a good simplification to the Kotlin/Java interoperability helpers. It replaces four specific callback completion functions with a single, more flexible generic function completeWithValue. This change reduces code duplication in ResultUtils.kt and simplifies the native Android implementation. The call sites in GoogleSignInPlugin.java are updated accordingly. The CHANGELOG.md and pubspec.yaml are also updated to reflect this internal improvement.

[stuartmorgan-g]

test-exempt: code refactor with no semantic change