Zen: Add notification policy management api.

 - Allow apps to read and modify notification policy (currently
   which items are prioritized in "priority only" mode), but only
   after they've been granted access by noman.
 - Access to read/modify notification policy requires a token provided
   by noman.  Enabled notification listeners are automatically given
   tokens (new getter on NLS), but any other app can also request them.
 - Currently, all requests are granted.
 - Also add a new change intent when the public policy changes.

Bug: 18541928
Change-Id: I482d1c39852d0d961931515e0f0e059a8faee4ed