Avoid looking up standard charsets in "java.desktop" module

This is a request to clean up a desktop module as was done in JDK-8233884 for "java.base" module.

In many places standard charsets are looked up via their names, for example:
absolutePath.getBytes("UTF-8");

This could be done more efficiently with use of java.nio.charset.StandardCharsets:
absolutePath.getBytes(StandardCharsets.UTF_8);

The later variant also makes the code cleaner, as it is known not to throw UnsupportedEncodingException in contrary to the former variant.