Disable bottom hit slop of iOS back button

[Manduro]

This fixes a bug where the hit area of the back button overlaps a few pixels of content below the header, making it impossible to interact with that content.

[enagorny]

Does it work fine on Android? As from the code I see it's bottom: 8,

[satya164]

Can you share a screenshot/gif showing the issue? We're not setting any hitSlop for iOS where this PR is adding extra hitSlop are, so I'm not understanding how it avoids extra area being overlapped

[Manduro]

On Android it works fine, the hit slop does not go outside of the header bar.

This shows the issue on iOS:

This PR is not adding extra hitSlop. There is a default hitSlop for iOS here: https://github.com/react-navigation/stack/blob/master/src/views/TouchableItem.tsx#L64
I merely took the default and set the bottom to zero.

[satya164]

Thanks for the clarification. Can we remove the extra hitSlop from TouchableItem instead? I think it's not needed and it's a bit confusing to have it by default.