fix the `updateNextRingText()` logic

- retrieve next ring text from AlarmStorage
remove the alarm storage entry after ring
fix json problem in AlarmMeta and AlarmStorage

- in constructor of AlarmMeta, if null is passed as weekdays, use
  an array of `false` to replace it.
- in AlarmStorage, when encoding an alarm to json, explictly build
  the weekdays JSONArray.