[Java]: Default tmpfs paths should support multi-user environments

Simple modifications to the default path (prefixes) to include the
user.name after “aeron” in order to avoid conflict in multi-user
environments.
- DATA_DIR_PROP_DEFAULT
- ADMIN_DIR_PROP_DEFAULT
- COUNTERS_DIR_PROP_DEFAULT

Example on Linux:
Old: /dev/shm/aeron/{conductor, counters, data}
New: /dev/shm/aeron/kris/{conductor, counters, data}

C++ support forthcoming.