[UIKit] Fixes UIGestureRecognizer default introduced in iOS 13.4

Fixes xamarin/Xamarin.Forms#10162
Fixes xamarin#8255

Xcode 11.4 introduced a new protocol member to
`UIGestureRecognizerDelegate` and our initial proposed default value for
`ShouldReceiveEvent` is not playing well with the world.

src/uikit.cs

@@ -6113,7 +6113,7 @@ interface UIGestureRecognizerDelegate {
 		bool ShouldReceivePress (UIGestureRecognizer gestureRecognizer, UIPress press);
 
 		[TV (13,4), iOS (13,4)]
-		[Export ("gestureRecognizer:shouldReceiveEvent:"), DelegateName ("UIGesturesEvent"), DefaultValue (false)]
+		[Export ("gestureRecognizer:shouldReceiveEvent:"), DelegateName ("UIGesturesEvent"), DefaultValue (true)]
 		bool ShouldReceiveEvent (UIGestureRecognizer gestureRecognizer, UIEvent @event);
 	}