simpleperf: monitor cpu hotplug in stat cmd.

Move code used to monitor cpu hotplug events
from cmd_record.cpp to event_selection_set.cpp,
so it can also be used by stat cmd.

Bug: http://b/29245608
Change-Id: I1f5f3268192aa5c98c0be9fc1de763352c08eab6
Test: run simpleperf stat, and online/offline a cpu.
4 files changed