Don't rely on rootDir for Javadoc gradle files

Issue #4059

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191419955

javadoc_combined.gradle

@@ -11,7 +11,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-apply from: "${rootDir}/javadoc_util.gradle"
+apply from: "${buildscript.sourceFile.parentFile}/javadoc_util.gradle"
 
 class CombinedJavadocPlugin implements Plugin<Project> {
 

javadoc_library.gradle

@@ -11,7 +11,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-apply from: "${rootDir}/javadoc_util.gradle"
+apply from: "${buildscript.sourceFile.parentFile}/javadoc_util.gradle"
 
 android.libraryVariants.all { variant ->
     def name = variant.buildType.name