This improves on b68640c4 "Prevent creation of
new ORequestThreads during shutdown," which added throwing the DisposedException
from ThreadAdmin::add.  But ThreadAdmin::m_disposed can only become true via
uno_threadpool_destroy -> ThreadPool::joinWorkers -> ThreadAdmin::join, and
ThreadAdmin::add observing that can only happen via uno_threadpool_putJob ->
ThreadPool::addJob -> ThreadPool::createThread -> ORequestThread::launch ->
ThradAdmin::add, where the bridges should ensure that uno_threadpool_destroy
does not run in parallel with uno_threadpool_putJob.  So demote this from a
DisposedException to a SAL_WARN.

Change-Id: I3912ea077b7fa35827c41e82dd0a8f962ba412b6