Declare dependency versions inline instead of in our parent pom.

[copybara-service]

Declare dependency versions inline instead of in our parent pom.

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.