* Implement custom save locations for Android and Desktop
* Request write permission to save to external storage
* Fix race condition for custom saves/loads caused by autosaves
* Remove unnecessary WRITE_EXTERNAL_STORAGE permission for saving files
* Fix padding for custom save/load location buttons
* Use nullability checks as defined in coding style guide
* Use nullability checks as defined in coding style guide
* Use early return for readability
* Rename save/load completion callbacks for custom locations and implement error handling