Android presubmit test failing due to key deletion

The `oauth_token.txt` file was removed https://github.com/grpc/grpc/pull/17746 - core isn't using it anymore, but it is (was) still in use in this repository for setting a PR status message, see [here](https://github.com/grpc/grpc-java/blob/94e7339748da08ad7cc9af90d73553d59b0b53b5/buildscripts/kokoro/android.sh#L110)

@jtattermusch Our Android presubmit tests are now failing. Can the key temporarily be restored to GCS? If there's a preferred way of authentication that supersedes the auth token, I can follow up by migrating but it would be nice to have the presubmit working in the meantime.