jni_uno: string comparison

Change-Id: If093afb43f6ad8d4fcf6c0ec426eb48a6061ef3e
Reviewed-on: https://gerrit.libreoffice.org/11470
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>