Fix SET in Expo Go

[piaskowyk]

Summary

This pull request aims to address the lack of Shared Element Transition in Expo Go. The issue stemmed from the initialization process of SET. In Expo Go, Reanimated gets instantiated twice (once for the Expo client and once for the actual application). During initialization, REASharedTransitionManager attempts to swizzle certain methods from react-native-screens but aims to do so only once:

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
  ...
  _isConfigured = YES;
});

The variable _isConfigured was only set to true if the swizzling process completed successfully. The problem appeared when the second instance attempted to set up the screens integration, as it had already been done, resulting in the new instance failing to set _isConfigured to true, even if the method swizzling was successful.

The solution involves changing _isConfigured to a class field instead of an instance field.

Test plan

Run Shared Element Transition in Expo Go

[piaskowyk]

@EvanBacon - fixed, thanks for the help yesterday 🤝