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

Change-Id: I05fafb903ae54e39c03a048b7a219dc5a93fd472