Previously, PerformanceTracker registered a callback to be updated on
emulation state changes. PerformanceTrackers live in a global variable
(g_perf_metrics) within libvideocommon. The callback was stored in a
global variable in libcore. This created a race condition at shutdown
between these libraries, when the PerfTracker's destructor tried to
unregister the callback.
Notify the PerfTracker directly from libcore, without callbacks, since
Core.cpp already references g_perf_metrics explicitly. Also rename
Core::CallOnStateChangedCallbacks to NotifyStateChanged to better
reflect what it's doing.
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