core,okhttp,testing: add proguard flags

[carl-mastrangelo]

@ericgribkoff had suggesting these flags files could live in OSS, and in the scope of #5422 it seems there would be some use of them.

Currently these have no effect, but they serve as references we can point people to. Also, these match the internal copies (in content and file path).

[ejona86]

Having the files doesn't bother me, other than they are useless/unused. This doesn't seem to help people. We have something similar to this already in https://github.com/grpc/grpc-java/blob/master/examples/android/helloworld/app/proguard-rules.pro . Although we could maybe enhance the one at https://github.com/grpc/grpc-java/blob/master/android/proguard-rules.txt and then encourage OkHttp users to use the android artifact.

[carl-mastrangelo]

Alright, I think this would simplify our import script slightly, so if you don't have any other concerns I would like to commit this. In the future we can consolidate the proguard-rules.txt file with the ones here as you suggested.

[ejona86]

It will only be confusing externally. I would be happy to combine the internal and external approaches, but dumping this externally when unused is not a winner.

These will need to be changed when we do imports, and since they aren't used they will be out-of-date, causing another PR to sync the fix back to github.