Make "Undo" button text not bold
Update class comments
Update `NumberPickerPreference`

Use `int` instead of `Integer`, and rename `initialVal` to `value`
Move preference-related classes to new package
Rename layout files

To use “fragment_” and “pref_” prefixes
Add license header to file
Use different method to detect OK button in preference

This is the way shown in the guide:
http://developer.android.com/guide/topics/ui/settings.html#CustomSave
Call `setVolumeControlStream()` in MainActivity
Add volume preference

Allow user to adjust the alarm volume from `SettingsActivity`