grpc-java contributors: Android projects integrated into main build

[Eric Anderson]

I'm about to merge https://github.com/grpc/grpc-java/pull/6132. This will add the Android projects to our main project, so they will build by default now.

If you have an Android SDK, I'd encourage you to build/test the Android projects as part of the normal development process. However, if you don't have Android SDK or don't like them, you can use -PskipAndroid=true (similar to -PskipCodegen=true) to avoid this new build dependency. I've updated COMPILING.md to note as much.

Today only Cronet will be built. But in the future we expect to add android-interop-testing/ and android/ to our main build process.

If you have trouble, feel free to email this list and CC me or we can discuss on Gitter.