And make some tools::Time conversion ctors explicit to catch more oddities like
automatic conversion from sal_Int64 that might be unintentional.

Change-Id: If275297d86d6657544c056a712b862523e310e44
Reviewed-on: https://gerrit.libreoffice.org/40275Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>