Zen: Store notification zen policy per-user.

 - Keep a zen config per user in NoMan.
 - Add zen config for all users to xml policy storage mechanism.
 - Initialize config to default for new secondary users.
 - Re-evaluate global zen on user switch.
 - Remove some unused code in NoMan.
 - Make ZenModeHelper aware of multiple users, keep all configs,
   add to dump.
 - Log config diffs in addition to the config itself in ZenLog.

Bug: 15845915
Change-Id: Ic847451e5d111c74916def1ea0948db5a76966c9