Summary:
This adds support for sticky headers on Android. The implementation if based primarily on the iOS one (https://github.com/facebook/react-native/blob/master/React/Views/RCTScrollView.m#L272) and adds some stuff that was missing to be able to handle z-index, view clipping, view hierarchy optimization and touch handling properly.
Some notable changes:
- Add `ChildDrawingOrderDelegate` interface to allow changing the `ViewGroup` drawing order using `ViewGroup#getChildDrawingOrder`. This is used to change the content view drawing order to make sure headers are drawn over the other cells. Right now I'm only reversing the drawing order as drawing only the header views last added a lot of complexity especially because of view clipping and I don't think it should cause issues.
- Add `collapsableChildren` prop that works like `collapsable` but applies to every child of the view. This is needed to be able to reference sticky headers by their indices otherwise some subviews can get optimized out and break indexes.
Closes https://github.com/facebook/react-native/pull/9456
Differential Revision: D3827366
fbshipit-source-id: cab044cfdbe2ccb98e1ecd3e02ed3ceaa253eb78
0e8b75b22c