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

src/com/android/settings/wifi/slice/WifiSlice.java

@@ -105,8 +105,8 @@ public class WifiSlice implements CustomSliceable {
             return listBuilder.build();
         }
 
-        final List<AccessPoint> results =
-                SliceBackgroundWorker.getInstance(mContext, this).getResults();
+        final SliceBackgroundWorker worker = SliceBackgroundWorker.getInstance(getUri());
+        final List<AccessPoint> results = worker != null ? worker.getResults() : null;
 
         // Need a loading text when results are not ready.
         boolean needLoadingRow = results == null;