Restrict checkUpperBoundDeps to library dependencies

Guava 32.1.0 added Gradle Module Metadata to their artifact (they still
use Maven for their build). Because of that, we're now seeing less
normal dependencies that were confusing checkUpperBoundDeps.  Obviously
'null' is not the version we were looking for.

We aren't upgrading to Guava 32.1.x yet as there's still other problems.

```
Execution failed for task ':grpc-census:checkUpperBoundDeps'.
> Maven version skew: com.google.guava:guava-parent (32.1.1-jre != null) Bad version dependency path: [project ':grpc-census', com.google.guava:guava:32.1.1-android] Run './gradlew :grpc-census:dependencies --configuration runtimeClasspath' to diagnose
```

build.gradle

@@ -504,6 +504,11 @@ def requireUpperBoundDepsMatch(Configuration conf, Project project) {
                 + "to diagnose")
         }
         result.selected.dependencies.each {
+            // Category.CATEGORY_ATTRIBUTE is the inappropriate Attribute because it is "desugared".
+            // https://github.com/gradle/gradle/issues/8854
+            Attribute<String> category = Attribute.of('org.gradle.category', String)
+            if (it.resolvedVariant.attributes.getAttribute(category) != Category.LIBRARY)
+                return
             queue.add(new DepAndParents(
                 dep: it, parents: depAndParents.parents + [artifact + ":" + version]))
         }