Consider merging _ForceAdvance and _NativeFixedLengthTimer vars

[KrisV-777]

I question if we need this variable or could just use ForceAdvance, or alternatively remove _ForceAdvance. Since _ForceAdvance is set by timed stages, and _NativeFixedLengthTimer is for timed stages, they seem to ultimately have the same functionality. Especially when we consider that every call to update the timer sets _NativeFixedLengthTimer to _ForceAdvance

In case one of them I can be removed, Im in favor of keeping _NativeFixedLengthTimer, as its more expressive

Originally posted by @KrisV-777 in #43 (comment)