Fix all warnings in jpackager java code

In make/CompileJavaModules.gmk:
We use GENERATE_JDKBYTECODE_NOWARNINGS
 When you add new code to OpenJDK, it really *should* be able to build without warnings. This has previously only been used for legacy code that has not yet been brought up to OpenJDK standards. Introducing new code with warnings silenced feels like a step in the wrong direction.

we need to remove the GENERATE_JDKBYTECODE_NOWARNINGS and fix all the code that generates warnings