Fix binder proxy death notice tracking

There was an issue with stale recipient tracking when BinderProxy weak
references had been purged and a new proxy object allocated for a
still-live underlying IBinder.  The death recipient bookkeeping has
now been reworked so that it's fundmentally tied to the BinderProxy
instances, not maintained as global state, to prevent this sort of
confusion entirely.

Bug 3499939

Change-Id: I75c5216b6d53b90868ac969e32c9725201e51be3