javaunohelper: use HashMap instead of Hashtable Collection

Change-Id: I04e3245342a83df762fbec35a1523021a021f896
Reviewed-on: https://gerrit.libreoffice.org/11547
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>