init: Improve readability of the code in SecondStageMain()

Change the type of epoll_timeout from
std::optional<std::chrono:milliseconds> into std::chrono::milliseconds.
No functionality is changed.

Bug: 213617178
Change-Id: Ieb150e0aeabdb79c8da4649875ea3ed59297343b
Signed-off-by: Bart Van Assche <bvanassche@google.com>