Implement Android shims in Swift instead of C

[madsodgaard]

#1663 introduced shims for getgrnam_r and getgrgid_r. Those are written in C, because even though we added #available support in Swift 6.3 for Android, we still need to bump the NDK version from 27 to 28, to actually reap the benefits when interacting with C APIs.

So, when the official Android SDK/CI uses NDK 28+ instead, we should rewrite those shims to Swift to match the goal of the swift-foundation project, of preferring Swift implementations.