Fix HUN unbind crash when notif is removed.

There appear to be some edge cases where we expect HUN entries to live
longer than the NotificationEntryManager entries so it's possible to
trigger an "unbind" operation after the entry is removed and trigger
an illegal state exception. This can be solved with a check to see if
the notification has been removed.

Bug: 153902989
Test: builds
Change-Id: I6a034e07d0983681fbbe01a8f25dce2881668a3f