Convert all AsyncTasks to Callables called from ReactiveX

public class ReadFileTask extends AsyncTask<Void, Void, ReturnedValueOnReadFile> {
to
public class ReadFileTask implements Callable<ReturnedValueOnReadFile> {

and used as

final ReadFileTask task = new ReadFileTask(getContentResolver(), viewModel.getFile(), getExternalCacheDir(), isRootExplorer());

final Consumer<ReturnedValueOnReadFile> onAsyncTaskFinished = (data) -> {
//operations
}

final Consumer<? super Throwable> onError = error -> {
  error.printStackTrace();
}
Flowable.fromCallable(task)
  .subscribeOn(Schedulers.io())
  .observeOn(AndroidSchedulers.mainThread())
  .subscribe(onAsyncTaskFinished, onError);

Use the ReadFileTask example and the Flowable.fromCallable entry point as the starting references. Locate the other AsyncTask classes in the project, then verify that each conversion preserves its result and error handling while using the ReactiveX flow shown in the issue. Done means no remaining AsyncTask implementations require conversion.