Add workflow that runs jacocoTestReport on AnkiDroid PR against Anki-Android-Backend PR

There was an issue that only turned up in Anki-Android (this repo) CI after merging + releasing an Anki-Android-Backend PR and it would be nice to have a workflow that allowed a PR here that was dependent on a PR there to run our CI targets on the PR here against the result of the backend build there

Just some brainstorming:
- I believe it is possible to have some workflows be callable and provide them arguments, perhaps we could keep the CI workflows fully separate in their repos and just call them - that is have a backend workflow we can call on a specific git commit hash and it will provide a local library we can test?
- maybe in the test workflows here, if there is a local.properties parse it and see if it is local_backend=true, if it is pull a new property that is a git reference (so you can use a branch name) and if it is, then call out to a backend workflow on that hash to build a local rsdroid ?

That would require a final commit in a PR here after launch+release to remove the local.properties but it would still be incremental improvement

...or something