[sdk-platform-java] Downstream check should include changes from the generator

We currently have [downstream check](https://github.com/googleapis/sdk-platform-java/blob/main/.kokoro/presubmit/downstream-compatibility.sh) that validates any changes in Gax or shared dependencies would not break downstream(java-bigtable etc.) libraries. However, it does not include changes from the generator, which caused the downstream libraries to fail. It happened recently for bigtable in the [generator update PR](https://github.com/googleapis/java-bigtable/pull/2191).
It was not possible to do so in the past due to BazelBot and Owlbot are not in this repo. Now that we have hermetic build, we can test downstream libraries with changes from the generator as well.