init/epoll: Fix a potential use-after-free

If UnregisterHandler() is called from inside a handler for an event that
has not yet been processed then that will result in a use-after-free.
Fix this by passing file descriptors to epoll_ctl() instead of pointers
to map elements.

Bug: 213617178
Change-Id: Ie62e3a299af964271ec24cd8fc2e794042b77ee6
Signed-off-by: Bart Van Assche <bvanassche@google.com>