util: Reorganize the java wrapper for gem5 ops.

util: Reorganize the java wrapper for gem5 ops.

Rather than use a top level package of jni which is generic, switch to a
top level package of "gem5". With that prefix, call the actual class
Ops, which is capitalized according to Java tradition and also
unambiguous given its package name.

Also move the java class definition and c JNI implementation into a java
subdir to keep it all together. The java related output will now be in
out/java for the same reason.

Change-Id: Ia0468d2edbcffe87a62022898f867ae391adc94c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28176
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>