accessibility: Calling String.toString()

Change-Id: I125d2c337df606c6388ddbd41030e51a558ad054
Reviewed-on: https://gerrit.libreoffice.org/11920
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>