Use std::shared_ptr in Epoll's callback list. am: 2cf268ab9f am: 81c5a30224 am: bbedda71e1

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/15128087

Change-Id: I833f5fceb7db60dcebbdb49a0a8e5af0c55fea9c