Add basic alarm storage logic in SettingActivity

- add a storage entry when an alarm is set
- read storage in `onCreate()`

Side effect:
- find that when back button is clicked some exception is throwed.
  after some searching, 2 facts of `onPause()` are founded:
    1. any existing broadcast register should be unregistered
    2. once the `onPause()` (and other life cycle related methods)
       is explictly overrided, the `super.onPause()` needs to be
       called.