Mappings Between Android Architectures and Bazel CPU names

Hi wonderful Bazelers,

I'd like to float an idea for making official something I find myself repeatedly rebuilding on top of platforms when I use it with the Android rules.

Android has its own names for CPU architectures that differ slightly from the Bazel CPU names over in https://github.com/bazelbuild/platforms. In order to use libraries, including Google ones, like Firebase, or to otherwise build Android things in Starlark, you have to hardcode that mapping.

It'd be great if rules_android would could be the source of truth for that mapping at some point in the future! I know there's lots left to do before a v1 gets out, but I wanted to chime in that I think this is going to be a generally helpful feature more broadly than just being needed for the implementation of the starlark rules.

The mapping could look something like:
```Starlark
BAZEL_CPU_ANDROID_ARCH_PAIRS = (
("armv7", "armeabi-v7a"),
("arm64", "arm64-v8a"),
("x86_32", "x86"),
("x86_64", "x86_64"),
)
```

Happy to contribute of course--and would have made this a PR--except that I'm not sure quite how this repo is being managed wrt the starlark rules alpha branch.

Cheers!
Chris
(ex-Googler)