Use onUnlinked in health HAL

It's possible to get an onBinderDied callback after a call to
AIBinder_unlinkToDeath() so we can't delete the objects in callbacks_
until we are done using the void* cookie.
Handling the cleanup in onBinderUnlinked will handle the case where we
manually unlink it as well as the case where it's unlinked due to death.

Test: atest VtsHalHealthTargetTest
Bug: 319210610
Change-Id: Iee4783217cc88134af6de0fe66128684ca984dba