removeFile do not remove my file

## 🐛 Bug Report

I am trying to remove a specific file from the cache and clear cache too.

### Expected behavior

(The problem only occurs in Android)

My customer take a picture with the application and he can retry to take another one if the first is not good, so I want to remove the previous picture from the cache folder of the application :
/data/user/0/*package_name*/cache/customCache/*file_name*

But nothing happens when I call the function from my CustomCacheManager class.

### Reproduction steps

This is my CustomCacheManager like your example :

```
class CustomCacheManager extends BaseCacheManager {
static const key = "customCache";

static CustomCacheManager _instance;

factory CustomCacheManager() {
if (_instance == null) {
_instance = new CustomCacheManager._();
}
return _instance;
}

CustomCacheManager._() : super(key,
maxAgeCacheObject: Duration(days: 2),
maxNrOfCacheObjects: 20);

@override
Future getFilePath() async {
var directory = await getTemporaryDirectory();
return join(directory.path, key);
}

Future removePicture(String file) async {
await removeFile(file);
}

Future clearCustomCache() async {
await emptyCache();
}
}
```
I am trying to remove the picture like that :
```
retryTakePicture(String imageToRemove) async {
await _customCacheManager.removePicture(imageToRemove).then((_) {
***some code***
});
}
```

So when I retry to take the picture, the file is not removed from the cache so I got CameraException :
```CameraException(fileExists, File at path /data/user/0/*package_name*/cache/customCache/*file_name*' already exists. Cannot overwrite.)```

I don't know if I misunderstood the purpose of the removeFile method or if it's a bug.

And I have the same problem with `emptyCache()`.

### Configuration

**Version:** 1.2.2

**Platform:**
No problem on iOS

Start with the BaseCacheManager entry points removeFile and emptyCache, using the CustomCacheManager example in the issue to reproduce the behavior on Android. Check the cache path under the app's temporary directory and verify that removing one file and clearing the cache both complete before retryTakePicture runs. Done means the existing file is gone and the camera no longer reports fileExists.

索引モデルが issue の本文から書いたものです。