Look for const string fields which can be static, and
mostly convert them to OUStringLiteral

And add a getLength() method to OUStringLiteral to make
the transition easier.

Remove dead code in XclExpRoot::GenerateDefaultEncryptionData,
default password is never empty.

Change-Id: Iae75514d9dbb87289fd5b016222f640abe755091
Reviewed-on: https://gerrit.libreoffice.org/59204
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>