Make mPendingNotifications truly private (mk II)

(Added a nullcheck to callback in removeNotificationInternal)

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: I2bb345097b34fe18f008834c8f95fdb01f421436