Introduce com.sun.star.task.theJobExecutor singleton.

To replace com.sun.star.task.JobExecutor single-instance service,
incorrectly converted in 748aa84e9808ad31c6ff6b71459525c82de10e58

[including changes by Stephan Bergmann <sbergman@redhat.com>]

Change-Id: I4cea2c63a20b5b22f6e1f822fb35fcc4d0397687
Reviewed-on: https://gerrit.libreoffice.org/7609
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>