Fixes #6654, I hope? If `guava` and its parent pom don't refer to `mockito-core`, then `guava` should no longer affect which version of `mockito-core` is selected by Gradle. (Really, it "shouldn't" even now, but there's a mismatch between Maven's model and Gradle's that causes it to do so.)

As part of resolving a merge conflict in the `android` flavor, I also noticed that I had never added a direct `checker-qual` dependency to that flavor's `guava-testlib` or `guava-tests` configuration, as I should have done back in cl/522315614 or thereabouts. So I added it. (Of course, it matters little because Maven lets us use the `checker-qual` dependency of `guava` transitively.)

It would be nice if we could still declare our dependency versions only once, now by using `properties`. (In fact, my attempt to use `properties` made me notice that our version of the Error Prone _plugin_ is older than our version of the Error Prone _annotations_.) However, if we were to make that change, then we'd lose the ability to update dependencies with `versions-maven-plugin` (`update-properties` + `use-latest-releases`), I assume because the properties are declared in one `pom.xml` and used in another. (It's possible that Dependabot is better about this, but we've had trouble getting it to work with our unusual 2-flavor, Google-repo-source-of-truth setup.)

Tested:
  ```
  $ mvn dependency:tree -Dverbose -X -U &> /tmp/pre

  # made changes

  $ mvn dependency:tree -Dverbose -X -U &> /tmp/post

  $ strip() { cat "$@" | grep -v -e 'Dependency collection stats' -e 'Downloading from' -e 'Progress' -e 'Using connector' -e 'Using transporter' -e 'Using mirror' -e maven-dependency-plugin -e SUCCESS -e 'Total time' -e 'Finished at' | sed -e 's/ (.*managed from.*)//'; }; vimdiff <(strip /tmp/pre) <(strip /tmp/post)
  ```

RELNOTES=Changed our Maven project to avoid [affecting which version of Mockito our Gradle users see](#6654).
PiperOrigin-RevId: 552479838