Some more changes to make it possible to use Java 11 to compile

[jianglai]

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

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 compilation tasks. I don't think there is a
strong reason that we set -Werror anyway, so this commit removes it.

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This change is 

[mindhog]

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jianglai)

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java_common.gradle, line 105 at r1 (raw file):

    if (System.getProperty('no_werror') != 'true') {
        options.compilerArgs << "-Werror"
    }

I'm a little concerned about this. Typically we do -Werror in order to be very aggressive about keeping the code free of warnings. I'm not sure what else to do about it, though. At minimum we should resurrect this when we have all the necessary gradle support.