[Android] can not access res file with readFileRes

ReactNative version: 0.64.2 react-native-cli project

I had try to access with code:
```javascript
const RNFS = require('react-native-fs');
RNFS.readFileRes('abc_btn_borderless_material.xml','base64').then(console.log).catch(console.log)
```

It shows in logcat( both debug & release):

> [Error: Resource ID #0x0]

I had read the code seems nothing wrong:
https://github.com/itinance/react-native-fs/blob/master/android/src/main/java/com/rnfs/RNFSManager.java#L306

and build cache shows that the apk contains the resource:

> android/app/build/intermediates/runtime_symbol_list/debug/R.txt
> android/app/build/intermediates/runtime_symbol_list/release/R.txt

both relative path contains:

> int drawable abc_btn_borderless_material 0x7f070008

besides:
getResIdentifier will not work when access `ic_launcher.png` for it is a mipmap:

> int mipmap ic_launcher 0x7f0c0001

I will use readFileAssets to access package file instead, before this issue had solved