MemoryCallbackSystem ActiveChanged only invoked on first Add

[gifvex]

See: MemoryCallbackSystem.cs

This is an issue for cores that rely on the event handler to pass callbacks in:

https://github.com/TASVideos/BizHawk/blob/519e14aa1ad7a9d6df2edc7808c5ed687dfee046/BizHawk.Emulation.Cores/Consoles/Nintendo/Gameboy/Gambatte.IDebuggable.cs#L74-L84

For example, if onmemoryexecute is called from lua, it will run RefreshMemoryCallbacks with only HasExecutes as true. Any calls to onmemorywrite or onmemoryread will still register the functions on the lua end, but the function pointer for those collections will never be passed to the core.