Refactored subview management

Summary:
This diff refactors the view update process into two stages:

1. The `reactSubviews` array is set, whose order matches the order of the JS components and shadowView components, as specified by the UIManager.
2. The `didUpdateReactSubviews` method is called, which actually inserts the reactSubviews into the view hierarchy.

This simplifies a lot of the hacks we had for special-case treatment of subviews: In many cases we don't want to actually insert `reactSubviews` into the parentView, and we had a bunch of component-specific solutions for that (typically overriding all of the reactSubviews methods to store views in an array). Now, we can simply override the `didUpdateReactSubviews` method for those views to do nothing, or do something different.

Reviewed By: wwjholmes

Differential Revision: D3396594

fbshipit-source-id: 92fc56fd31db0cfc66aac3d1634a4d4ae3903085

Examples/UIExplorer/UIExplorer/NativeExampleViews/FlexibleSizeExampleView.m

@@ -90,6 +90,7 @@ RCT_EXPORT_MODULE();
 - (NSArray<UIView<RCTComponent> *> *)reactSubviews
 {
   // this is to avoid unregistering our RCTRootView when the component is removed from RN hierarchy
+  (void)[super reactSubviews];
   return @[];
 }
 

Examples/UIExplorer/UIExplorer/NativeExampleViews/UpdatePropertiesExampleView.m

@@ -89,6 +89,7 @@ RCT_EXPORT_MODULE();
 - (NSArray<UIView<RCTComponent> *> *)reactSubviews
 {
   // this is to avoid unregistering our RCTRootView when the component is removed from RN hierarchy
+  (void)[super reactSubviews];
   return @[];
 }