Refactor Android filesystem platform helpers

[kraenhansen]

What, How & Why?

This closes #5296 using the std::filesystem API as a pre-requisite for merging https://github.com/realm/realm-js/pull/6927 (as that needs the ensure_directory_exists_for_file to get tests passing).

☑️ ToDos

• [x] 📝 Changelog entry
• [ ] 📝 Compatibility label is updated or copied from previous entry
• [ ] 📝 Update COMPATIBILITY.md
• [ ] 🚦 Tests
• [ ] 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• [ ] 📱 Check the React Native/other sample apps work if necessary
• [ ] 💥 Breaking label has been applied or is not necessary
• [ ] 🔔 Mention @realm/devdocs if documentation changes are needed

[kraenhansen]

Ran tests locally to verify this, Android is green and because of https://github.com/realm/realm-js/commit/93d7d3e4548e534792522b17b0eeeace28d5e101 this will never be green on CI.