Pipe notifications from related users to listeners.

For Listeners built against L or greater
Send notifications from related users to listeners.
Return notifications from related users getAllActiveNotifications
Cancel notifications from related users in cancelAllNotifications
Deprecate StatusBarNotification.getUserId() and expose getUser()
as APIs should use UserHandles.
Deprecate cancelNotification that takes package, id and tag
in favour of one that takes key.

Fix bug that notifications from related users didn't
trigger sounds.

Change-Id: I1b1c20c9f305b8f3c4047bc5720d8e99cdedfe70