screen freezes on Android when reading file

Hi,

when running following code on Android the UI freezes while reading the file.

```
console.log("LOAD from file");
const contents = await RNFS.readFile(RNFS.DocumentDirectoryPath+"/myfile.data");
```

The file is rather large (10MB). I read the note on readFile:
> Note: you will take quite a performance hit if you are reading big files

But how can I run the reading in the background?

In iOS the screen does not freeze or only freezes for a short period as the reading happens much faster.