Support contentContainerStyle paddingTop

[jsamr]

Currently, contentContainerStyle's paddingTop is overriden by header's height.

I suggest either:

• Add a new prop, such as extraPaddingTop which will be added to headerHeight to infer top padding
• Use Stylesheet.flatten to evaluate if user provided a paddingTop, and add this padding top toheaderHeight to infer top padding.

[iyegoroff]

Hi @jsamr , sorry for late reply - I just don't have enough time right now to revisit this. Please feel free to implement this feature the way you prefer and open a PR.