Some more changes to make it possible to use Java 11 to compile (#732)

Add the class paths of the source files generated by annotation processors to
the javadoc task's class path so that it doesn't complain about missing
Dagger classes.

Also remove empty <p> tags in all generated source files, because jaxb
genrerates files in multiple locations.

Lastly, for unkown reasons when the source level is set to > 8, the core
subproject throws a warning about a Gradle internal annotator processor
that only supports up to Java 8 and cause the Java compliation to fail because
we set -Werror on all java compliation tasks. I don't think there is a
strong reason that we set -Werror anyway, so this commit removes it.

core/build.gradle

@@ -404,7 +404,9 @@ task jaxbToJava {
         }
       }
     }
-    execInBash('find . -name *.java -exec sed -i /\\<p\\>\\$/d {} +', outputDir)
+    execInBash(
+        'find . -name *.java -exec sed -i /\\*\\ \\<p\\>\\$/d {} +',
+        generatedDir)
   }
 }