Android rules can't use external repo for toolchain

Currently, we require developers to install a package with the SDK and NDK. This makes upgrades almost impossible to version and track.

I want to be able to tar up the SDK and NDK and point Bazel to them. An external repo would be awesome and bazely. A URL that the rules fetch from and unpack would also work.

It looks like we *might* be able to check the toolchains into our repo and it would work that way. That would blow the size of our repo up in a way that's not scalable long term.