[ReactNative] Native touch unique identifier

Summary:
This bug was causing a redbox when touching rapidly because multiple touches had the same identifier. This code was meant to ensure that a unique ID is found, but it was checking against the wrong property in the react touch array.

I don't think this was really affecting prod, because the invariant is guarded by a __DEV__ check

@public

Test Plan:
- Start several touches at once (or just rapidly jam fingers on your device), and you won't see a redbox
- Also verify that single touches, touch moves, and multi touches work as before

React/Base/RCTTouchHandler.m

@@ -89,11 +89,11 @@ typedef NS_ENUM(NSInteger, RCTTouchEventType) {
       return;
     }
 
-    // Get new, unique touch id
+    // Get new, unique touch identifier for the react touch
     const NSUInteger RCTMaxTouches = 11; // This is the maximum supported by iDevices
-    NSInteger touchID = ([_reactTouches.lastObject[@"target"] integerValue] + 1) % RCTMaxTouches;
+    NSInteger touchID = ([_reactTouches.lastObject[@"identifier"] integerValue] + 1) % RCTMaxTouches;
     for (NSDictionary *reactTouch in _reactTouches) {
-      NSInteger usedID = [reactTouch[@"target"] integerValue];
+      NSInteger usedID = [reactTouch[@"identifier"] integerValue];
       if (usedID == touchID) {
         // ID has already been used, try next value
         touchID ++;