android_interop_testing frequently failing

We used to see infrastructure failures, but it seems those have gone away. Now I'm seeing timeouts after 60 seconds, which doesn't give us much to go on.

We are hitting a live server (grpc-test.sandbox.googleapis.com). I wonder if we should instead start a local server, now that we have OkHttp server available. I found one clear example of network connectivity being a problem ["ConnectException: Connection timed out"](https://firebase.corp.google.com/u/0/project/grpc-testing/testlab/histories/bh.6e2cfdde264f9eb3/matrices/9007317761909722027).

The failure is generally for the Nexus6P. And API levels 29 and 30 from https://github.com/grpc/grpc-java/pull/10461 see frequent failures.