Prior to this change, it's possible for m_wake_me_up_again to be used
while it's in an uninitialized state from the exposed API.
e.g.
- Using SetEnable after construction would perform an uninitialized read.
- Using PushEvent would perform an uninitialized read by way of operator |=.
internally, an uninitialized read can happen if PullEventsInternal() is
executed before other functions.
Just to avoid the whole possibility of performing uninitialized reads,
we just give the class member a default value of false.
Replaces old and simple usages of std::atomic<bool> with Common::Flag
(which was introduced after the initial usage), so it's clear that
the variable is a flag and because Common::Flag is well tested.
This also replaces the ready logic in WiimoteReal with Common::Event
since it was basically just unnecessarily reimplementing Common::Event.
