Fix warnings of JDWP tests

This CL fixes general warning issues:
- Adds missing @Override annotations
- Adds parameterized types for generic classes (List, Iterator, ...)
- Removes unused imports
- Removes declared method exceptions which are not thrown
- Adds @SuppressWarning annotations in debuggee classes (like when a
  symbol is not used in the code but is accessed through debugger)
- Makes inner classes static when possible

More specific issues (related to design, ...) will be addressed in
separate CLs.

Test: art/tools/run-jdwp-tests.sh --mode=host --variant=X64
Change-Id: I83709f5d76b2a8c9268cd11dd7884982d5d06f2d