AndroidTestVariantClasspathJar

A KeeperJarTask task that sources from both the androidTest compiled sources and its distinct dependencies (as compared to the appJarsFile). R8's TraceReferences requires no class overlap between the two jars it's comparing, so at copy-time this will compute the unique androidTest dependencies. We need to have them because there may be APIs that they use that are used in the target app runtime, and we want R8 to account for those usages as well.