init/epoll: Stop using smart pointers

Change the type of 'handler' from std::shared_ptr<Handler> into Handler.
This change is safe since a previous CL moved the handler calls from the
Epoll::Wait() caller into Epoll::Wait() itself.

Bug: 213617178
Change-Id: Ife79e6863536b96ee4bb3cd778f6b0b164a95fed
Signed-off-by: Bart Van Assche <bvanassche@google.com>