Missing launch of FactoryLoader threads

...regression introduced with 06a09dd0ad1e8afcf69e877501036e068d477438 "Join
FactoryLoader threads."  (Also, hopefully, the original use of
osl::Thread::setPriority was just cargo cult, as salhelper::Thread does not make
that feature available to clients, so it had been dropped in
06a09dd0ad1e8afcf69e877501036e068d477438.)

Change-Id: I055c8fb20f3bbf13ba823436be24167d3446ed05