Fix ConcurrentModificationException in SliceBackgroundWorker

- Two different threads could call SliceBackgroundWorker.getInstance()
  at the same time and caused ConcurrentModificationException
- Add a new API overloading getInstance for each slice to get a nullable
  worker since there is no result data then
- Only slice provider can create a new worker instance in main thread

Test: robotest
Change-Id: I560529bb6034ec22263418adeb7f3ccebf879196
Fixes: 121043385