Writes all modified preferences to disk.
By default Unity writes preferences to disk during OnApplicationQuit().
In cases when the game crashes or otherwise prematuraly exits,
you might want to write the PlayerPrefs at sensible 'checkpoints' in your game.
This function will write to disk potentially causing a small hiccup, therefore it is not recommended to call during actual gameplay.
Note: There is no need to call this function manually inside OnApplicationQuit().
Note: On Windows Store Apps and Windows Phone 8.1 preferences are saved during application suspend, because there's no application quit event.