docs: use safe casts in Android Gradle signing configuration

[lamek]

Updates the Android build.gradle.kts release signing example to use safe casts (as? String) instead of unsafe casts (as String). This prevents Gradle configuration from crashing on debug builds when key.properties is absent.

Fixes #11995

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[gemini-code-assist]

Code Review

This pull request refines the Android deployment documentation by improving phrasing, punctuation, and formatting, as well as updating a Kotlin Gradle configuration snippet to use safe casts (as? String) instead of unsafe casts. The reviewer suggests a further improvement to use the standard getProperty(key) method on the Properties object, which is more idiomatic and avoids the need for casting.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit ce4b2f4):

https://flutter-docs-prod--docs-pr13743-ticket-542694649-issue-3x37sphp.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit ce4b2f4):

https://flutter-dev-230821--www-pr13743-ticket-542694649-issue-f13zl06g.web.app

[sfshaza2]

Please use a more descriptive title for the PR. thx

[lamek]

@sfshaza2 updated with your feedback. Thanks for the review.