Simplify NotificationListenerService API

Use ParceledListSlice to send the list of active notifications
from NoMan to NotificationListeners.

This allows us to simplify the API to what it was before
introducing the "String[] key" workaround for dealing with large
numbers of StatusBarNotifications.

While we're at it, rename Ranking.getIndexOfKey to something that
makes more sense.

Bug: 15126927
Change-Id: I02c2087978c6c4ec1198be496c6250a66143ecb3