Add 'public' modifier to getResult method in RunnableClosure.java.

Kawa docs (Threads section) suggest this method should be callable externally. Based on some light testing, it seems to work as advertised after this change.

[Sorry for the messy commits. I messed up with the gitlab interface and my rebasing did not work as I had hoped. It may be easier to just make this trivial change separately in the main repository instead of merging it in.]