It is more efficient to use the integer implementations of String.indexOf()

Change-Id: Idb1e4945088b8443e9b507826e5289ee758a2bd2
Reviewed-on: https://gerrit.libreoffice.org/11919
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>