Add Android CI

[finagolfin]

I just cross-compiled this package with the Dec. 5 snapshot of the Swift SDK for Android without a problem, then natively compiled it with a recent trunk snapshot toolchain and ran its tests: all passed.

The official Android workflow doesn't support running tests yet, but we can at least make sure this repo keeps cross-compiling, until that testing is available too.

[czechboy0]

Yup we should add that - happy to take a PR

[finagolfin]

@marcprux is adding testing in an Android emulator to the official workflow now, swiftlang/github-workflows#215, we'll submit that check here once it's in.