[java] Make changes following Julien's review of my recent patch

java/runtime/JavaInternal.java:
java/runtime/MercuryRuntime.java:
java/runtime/MercuryThreadPool.java:
samples/java_interface/standalone_java/JavaMain.java:
samples/java_interface/standalone_java/Makefile:
samples/java_interface/standalone_java/mercury_lib.m:
    As above.