[Android] assetExistsRes doesn't check for extension

`assetExistsRes` breaks if you give it a file without extension with an index out of range error. The bug is [here](https://github.com/itinance/react-native-fs/blob/57b6131aaad5b7721857b9bf24dc86d8bcaf6eda/android/src/main/java/com/rnfs/RNFSManager.java):

```
private int getResIdentifier(String filename) {
String suffix = filename.substring(filename.lastIndexOf(".") + 1);
String name = filename.substring(0, filename.lastIndexOf("."));
// ...
}
```

The function should probably check that index before feeding it to `.substring`, and on that note it might be nice to provide an example of how to use `assetExistsRes` since users usually interact with files in `res` by removing the extension, it definitely isn't obvious that you have to give it the filename with extension.