Make mPendingNotifications truly private

Previously, NotificationEntryManager was exposing a reference to its
internal HashMap, mPendingNotifications, to
NotificationGroupAlertTransferHelper. NGATH only needed the reference to
iterate over the pending notifications, so now NEM exposes a new
getPendingNotificationsIterator() method that just exposes a read-only
iterator.

Also removes any references to NGATH in NEM. Initializes NGATH
independently from NEM.

Test: updated unit tests

Change-Id: Ic7471baa5a56737b9f5debaa30b0b51ef8bdb97d