Modify the preferences asynchronously

Using `SharedPreferences.Editor#commit()` saves the preferences synchronously. Using `SharedPreferences.Editor#apply()` is asynchronous. The [documentation](http://developer.android.com/reference/android/content/SharedPreferences.Editor.html#apply()) points out that it's safe to replace calls to `#commit()` with calls to `#apply()` if already ignoring the return value.