Use std::shared_ptr in Epoll's callback list. am: 2cf268ab9f am: 81c5a30224 am: bbedda71e1 am: 4905ce5086

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

Change-Id: I0a9321f57e57ab9e6cdfcd7aca62f94c101e9f8f