Zen: New user flow for requesting DND access.

 - User flow is now similar to requesting access to notification
   content, namely prompting the user to visit a settings page
   for enabling/disabling apps access.
 - New ACTION_NOTIFICATION_POLICY_ACCESS_GRANTED_CHANGED intent
   for apps to listen to this state change.
 - Removed obsolete request method and associated internal callback
   aidl.
 - Added new android.permission.ACCESS_NOTIFICATION_POLICY permission
   for apps to include as a signal that they want to request this access
   (and therefore appear in the list on the settings page).
 - Improve javadocs, outline the user flow in NotificationManager#isNotificationPolicyAccessGranted
   and link to this method elsewhere.
 - NoManService now persists the user-enabled package list across reboots
   and does so per-user.
 - Rename public settings intent to correspond with the noman api.

Bug: 21621663
Change-Id: I72cbc21cd736e6a157b6be5d1d0ba0b4a8e7ef4e