No exist file after call 'getSingleFile'

## 🐛 Bug Report

I will be downloaded mp3 file from cloud storage.
Then used getSingleFile api and custom cache manager.

```dart
class CustomCacheManager extends BaseCacheManager {
factory CustomCacheManager() => _instance ??= CustomCacheManager._();
CustomCacheManager._()
: super(key,
maxAgeCacheObject: const Duration(days: 30),
maxNrOfCacheObjects: 200);

static const key = 'customCache';

static CustomCacheManager _instance;

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

```dart
final result = [];
final cacheManager = fileStorageRepository.getCacheManager;
for (var voice in voices) {
final file = await cacheManager.getSingleFile(voice.url);
print('${file.path} ${file.existsSync()}'); // file.existsSync() -> false
result.add(file);
}
return result;
```
It correctly get file but sometime file is no exist.
But It succeed if retry for a while.

Is cache setting incorrect or usage getSingleFile api incorrect?

Additional comment, attached error log with audio player in Android.
It occur error when play audio file getting from getSingleFile.

```
E/AudioplayersPlugin(26546): Unexpected error!
E/AudioplayersPlugin(26546): java.lang.RuntimeException: Unable to access resource
E/AudioplayersPlugin(26546): at xyz.luan.audioplayers.WrappedMediaPlayer.setSource(WrappedMediaPlayer.java:289)
E/AudioplayersPlugin(26546): at xyz.luan.audioplayers.WrappedMediaPlayer.setUrl(WrappedMediaPlayer.java:54)
E/AudioplayersPlugin(26546): at xyz.luan.audioplayers.AudioplayersPlugin.handleMethodCall(AudioplayersPlugin.java:65)
E/AudioplayersPlugin(26546): at xyz.luan.audioplayers.AudioplayersPlugin.onMethodCall(AudioplayersPlugin.java:44)
E/AudioplayersPlugin(26546): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:226)
E/AudioplayersPlugin(26546): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
E/AudioplayersPlugin(26546): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:631)
E/AudioplayersPlugin(26546): at android.os.MessageQueue.nativePollOnce(Native Method)
E/AudioplayersPlugin(26546): at android.os.MessageQueue.next(MessageQueue.java:336)
E/AudioplayersPlugin(26546): at android.os.Looper.loop(Looper.java:174)
E/AudioplayersPlugin(26546): at android.app.ActivityThread.main(ActivityThread.java:7356)
E/AudioplayersPlugin(26546): at java.lang.reflect.Method.invoke(Native Method)
E/AudioplayersPlugin(26546): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
E/AudioplayersPlugin(26546): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
E/AudioplayersPlugin(26546): Caused by: java.io.FileNotFoundException: /data/user/0/com.sample.app/cache/customCache/531acd30-cd4e-11ea-9fe9-5ba2b4995f64.mp3: open failed: ENOENT (No such file or directory)
E/AudioplayersPlugin(26546): at libcore.io.IoBridge.open(IoBridge.java:496)
E/AudioplayersPlugin(26546): at java.io.FileInputStream.(FileInputStream.java:159)
E/AudioplayersPlugin(26546): at android.media.MediaPlayer.setDataSource(MediaPlayer.java:1182)
E/AudioplayersPlugin(26546): at android.media.MediaPlayer.setDataSource(MediaPlayer.java:1160)
E/AudioplayersPlugin(26546): at android.media.MediaPlayer.setDataSource(MediaPlayer.java:1125)
E/AudioplayersPlugin(26546): at xyz.luan.audioplayers.WrappedMediaPlayer.setSource(WrappedMediaPlayer.java:287)
E/AudioplayersPlugin(26546): ... 13 more
E/AudioplayersPlugin(26546): Caused by: android.system.ErrnoException: open failed: ENOENT (No such file or directory)
E/AudioplayersPlugin(26546): at libcore.io.Linux.open(Native Method)
E/AudioplayersPlugin(26546): at libcore.io.ForwardingOs.open(ForwardingOs.java:167)
E/AudioplayersPlugin(26546): at libcore.io.BlockGuardOs.open(BlockGuardOs.java:252)
E/AudioplayersPlugin(26546): at libcore.io.ForwardingOs.open(ForwardingOs.java:167)
E/AudioplayersPlugin(26546): at android.app.ActivityThread$AndroidOs.open(ActivityThread.java:7255)
E/AudioplayersPlugin(26546): at libcore.io.IoBridge.open(IoBridge.java:482)
E/AudioplayersPlugin(26546): ... 18 more
E/MethodChannel#xyz.luan/audioplayers(26546): Failed to handle method call
E/MethodChannel#xyz.luan/audioplayers(26546): java.lang.IllegalArgumentException: Unsupported value: java.lang.RuntimeException: Unable to access resource
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.plugin.common.StandardMessageCodec.writeValue(StandardMessageCodec.java:278)
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.plugin.common.StandardMethodCodec.encodeErrorEnvelope(StandardMethodCodec.java:69)
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler$1.error(MethodChannel.java:236)
E/MethodChannel#xyz.luan/audioplayers(26546): at xyz.luan.audioplayers.AudioplayersPlugin.onMethodCall(AudioplayersPlugin.java:47)
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:226)
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
E/MethodChannel#xyz.luan/audioplayers(26546): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:631)
E/MethodChannel#xyz.luan/audioplayers(26546): at android.os.MessageQueue.nativePollOnce(Native Method)
E/MethodChannel#xyz.luan/audioplayers(26546): at android.os.MessageQueue.next(MessageQueue.java:336)
E/MethodChannel#xyz.luan/audioplayers(26546): at android.os.Looper.loop(Looper.java:174)
E/MethodChannel#xyz.luan/audioplayers(26546): at android.app.ActivityThread.main(ActivityThread.java:7356)
E/MethodChannel#xyz.luan/audioplayers(26546): at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#xyz.luan/audioplayers(26546): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
E/MethodChannel#xyz.luan/audioplayers(26546): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
```

### Expected behavior
Correctly get files.

### Reproduction steps
none

### Configuration

**Version:** 1.4.1

**Platform:**
- [x] :iphone: iOS
- [x] :robot: Android

Beginne mit der Untersuchung von BaseCacheManager.getSingleFile und dem überschriebenen getFilePath im gemeldeten CustomCacheManager und reproduziere anschließend den sporadisch auftretenden Fall auf Android mithilfe des bereitgestellten audio-player-Fehlers. Prüfe, ob der zurückgegebene Pfad vor der Wiedergabe noch existiert, und vergleiche das Ergebnis des Cache-Managers mit dem ENOENT-Pfad im Log. Als erledigt gilt die Aufgabe, wenn getSingleFile zuverlässig eine vorhandene, abspielbare Datei zurückgibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.