stronger checks when creating Java UNO components

check the visibility of construction method at load time for Java UNO
components. This means that we get more informative errors at startup,
instead of failures much later on when we actually create the component.

Change-Id: I08cd16875cf53cfeeef7e8954ecd0d3177d70eb5
Reviewed-on: https://gerrit.libreoffice.org/13446
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>