Notify StagedApexObservers when there is a change in set of staged APEX

The set of staged APEX is changed whenever:
- a session containing APEX succesfully passes pre-reboot verification
  and gets marked ready
- a staged session gets abandoned

Bug: 187444679
Test: atest StagingManagerTest
Change-Id: I5c5bb4523cdab46e22fe3c8e2373289d8619c10e