Refactor: Replace all anonymous classes with lambdas

### Actual behaviour
Right now, we have a lot of anonymous classes in the project.

### Expected behaviour
They can and should be converted to lambdas.

### Steps to reproduce
Eg. In method ```getFileList``` of ```SyncedFolderActivity``` we have two such cases.

### Environment data
Android version: NA

Device model: NA

Stock or customized system: NA

Nextcloud app version: NA

Nextcloud server version: NA

### Logs
#### Web server error log
NA

#### Nextcloud log (data/nextcloud.log)
NA

@AndyScherzinger Please review.