Change handler reference to weak pointer to prevent memleaks (#3987)

This PR fixes a memory leak by breaking a retain cycle between RNGestureHandler and RNManualActivationRecognizer. The RNGestureHandler owns the _manualActivationRecognizer with a strong reference, and previously the recognizer was holding a strong reference back to the handler, creating a cycle that prevented both objects from being deallocated.

Author: WoLewicki

packages/react-native-gesture-handler/apple/RNManualActivationRecognizer.m

@@ -2,7 +2,7 @@
 #import "RNGestureHandler.h"
 
 @implementation RNManualActivationRecognizer {
-  RNGestureHandler *_handler;
+  __weak RNGestureHandler *_handler;
   int _activePointers;
 }