update progress handling in PgpDecryptVerifyOperation

- less progress messages overall
- report progress again based on input stream position, if filesize is known
- limit progress messages to one every 200ms while decrypting
- also fixed a bug in DecryptListFragment, the recycler view now reuses
  old views as intended instead of crossfading all the time.

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/DecryptListFragment.java

@@ -169,7 +169,12 @@ public class DecryptListFragment
         vFilesList.setHasFixedSize(true);
         // TODO make this a grid, for tablets!
         vFilesList.setLayoutManager(new LinearLayoutManager(getActivity()));
-        vFilesList.setItemAnimator(new DefaultItemAnimator());
+        vFilesList.setItemAnimator(new DefaultItemAnimator() {
+            @Override
+            public boolean canReuseUpdatedViewHolder(@NonNull RecyclerView.ViewHolder viewHolder) {
+                return true;
+            }
+        });
 
         mAdapter = new DecryptFilesAdapter();
         vFilesList.setAdapter(mAdapter);