Inform libcore of time format pref. changes.

- Introduce a boolean extra for intent TIME_CHANGED that
  specifies if the user wants a 24 hour format or not.
- Have the ActivityManagerService inform running processes
  of changes to this preference.
- Add plumbing in ActivityThread to inform j.t.DateFormat

(cherry-picked from dd491cc756233c088fd26eba4918671fcc9cfc30)

Change-Id: Ib90636bda4bc8332cfa22def831877b524b5c486