Fix:STPAddCardViewController disappear immediately
Fix:STPAddCardViewController disappear immediately

Fix:STPAddCardViewController disappear immediately after is being presented.
Reason: `paymentRequestWithCardForm` is called in side another modalViewController's callback.
That block of callback is onModalHide which is Reactnative's built-in function.
There is a conflict that when onModalHide is called, we call paymentRequestWithCardForm which is presenting the STPAddCardViewController 
But, at the moment, the current Modal which is own onModalHide is still alive because of its memory is still being use by the onModalHide's block's content.
After paymentRequestWithCardForm is called, it mean we call `[RCTPresentedViewController() presentViewController:navigationController animated:YES completion:nil];` , then onModalHide() will return.
Then react-native framework will continue it job which is release the modalViewController, but not, the top presented is STPAddCardViewController.
So, I allow the onModalHide finish first by moving `[RCTPresentedViewController() presentViewController:navigationController animated:YES completion:nil];` to the end of main_queue.
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