lineage-sdk self-removing prefs: don't remove from death row until reaped

*) Graveyard is shared by all prefs with a given context.

*) When we reap in onAttached(), there may be prefs that aren't yet
   attached to the hierarchy.  Ensure that summonReaper keeps
   track of them until the last onAttached() is called in the context.

*) Fixes self-removing prefs that weren't being removed on a settings
   page that has at least one least other self-removing pref.

Change-Id: Ic48b86bb6f06aa29c44814a5487e2382349fb363