Allow consumers of logins-api android SDK to run unit tests that call functions implemented in Rust.

[thomcc]

This kind of sucks to do, but it's very hard for me to imagine that Lockbox will accept not being able to run unit tests (on-device tests in an emulator should still be possible, though, although this is a painful prospect).

Apparently we need to produce a jar file that contains both the java class files, and the compiled native code for every platform we want to support running tests on. I suspect in practice that means at least windows x86, x86_64, linux x86, x86_64, and macOS. Maybe linux arm/aarch64? Not sure.

This is a big pain. Cross compiling the rust shouldn't be a huge deal (hopefully? Supposedly rustup toolchain add can do everything), but I strongly suspect cross compiling OpenSSL/SQLcipher will suck (especially to windows and mac)

I suspect that the way to handling the native code dependencies in a sane manner will be to fetch precompiled versions.

Caveats there: On windows the ABI needs to match, and on all platforms we need to be sure to compile position independent code even though we're producing static libraries for our native code dependencies (for OpenSSL the only way I could find to build a static lib w/ PIC is to build a shared lib and grab the static lib it produces as an intermediate), since ultimately they're linked into a dynamic library.

[ncalexan]

Any update on this: we're pretty close to having this ready on the a-s side of the house. Publishing something for consumers to (well) consume isn't hard, but it would be nice to get #252 in place first.

We're in good shape for Linux and macOS, but Windows is sticky: as @thomcc seems to have already been aware of, the ABIs need to match -- and cross-compiling with mingw to a static library, a la #376, appears not to be ABI compatible with MSVC on my Windows 10 device. I think that we might be able to produce a DLL with mingw and use that with MSVC. That's OK here, I think, because this isn't actually going to ship to consumers. (On Android, we very strongly didn't want multiple .so files.)

[ncalexan]

OK, #425 has produced a logins-forUnitTests Maven publication that lets me run unit tests on macOS (and, Linux -- we already do that in automation).

What's left is figuring out how to make this uniform; it really spans https://github.com/ncalexan/rust-android-gradle/issues/13 and the very special configuration we need to cross-compile in Taskcluster (worked out in that PR). I honestly don't know the right division of labour between plugin, local repository, and automation scripts right now. I'm thinking about it and will try to put something together.

Then we'll need to publish more broadly -- hopefully via #252 but otherwise just with my personal bintray.