OkHttpTransportTest earlyServerClose_serverFailure_withClientCancelOnListenerClosed is flaky

Please answer these questions before submitting your issue.

### What version of gRPC are you using?
latest -> SNAPSHOT

### What did you expect to see?
All tests should pass. Instead I see this failure more than once and it seems to be a flaky test:

> Task :grpc-okhttp:test
io.grpc.okhttp.OkHttpTransportTest > earlyServerClose_serverFailure_withClientCancelOnListenerClosed FAILED
java.lang.AssertionError: Timed out waiting for server stream

build output:
https://source.cloud.google.com/results/invocations/3bc8b484-55e9-4e11-9476-c481f3dc1661/targets/grpc%2Fjava%2Fpresubmit%2Fmacos/log