Pass test_sources to Gradle template for extractPackages

[mhsmith]

See beeware/briefcase-android-gradle-template#60 (comment).

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

I've made a couple of minor tweaks of API usage that are equivalent, but more consistent with the rest of the codebase (pathlib instead of os.path; sys.platform rather than os.name) and added a changenote; but otherwise, looks good.

Passing through extract_packages as a pre-rendered string bugs me a little - outside of literal template inclusions like build_gradle_extra_content, my usual preference is to pass through raw data, rather than pre-rendered template content, so that the template can decide what to do with it. However, cookiecutter makes it painful to pass a list, so we'd need to do something like pass through a dictionary of all sources; while that would work, it feels like overkill. As this won't ever be directly modified by the user, and it's a property that we (through chaquopy) are in control ok, I'm OK with practicality over purity in this case.

[mhsmith]

Thanks: I meant to add a changenote but it looks like I forgot to save the file before committing.