Update shim for Java 15

Fix compilation/runtime issues on Java 15
Allows use with JustJ distro of JRE shipped with the IDE
Finally correctly pass working directory to flatpak-spawn
Add a workaround for JVMs configured via the directory
portal

Change-Id: I4646f9a770ad1ed3de4eab29c61810f5052e3174
Signed-off-by: Mat Booth <mat.booth@redhat.com>
Reviewed-on: https://git.eclipse.org/r/c/linuxtools/org.eclipse.linuxtools.eclipse-build/+/174720