Switch componentWillMount / componentWillUnmount order

Summary:
This switches our React feature flag so that componentWillMount happens before
componentWillUnmount when a child switches. It used to be inconsistent and this
makes it consistent and inline with what React Fiber does.

Breaking change. May cause issues.

Reviewed By: spicyj, bvaughn

Differential Revision: D4626543

fbshipit-source-id: f7eaf1ebd479ca9fada012f903a2f972a7901b40

Libraries/Renderer/src/renderers/shared/utils/ReactFeatureFlags.js

@@ -17,7 +17,7 @@ var ReactFeatureFlags = {
   // render (both initial renders and updates). Useful when looking at prod-mode
   // timeline profiles in Chrome, for example.
   logTopLevelRenders: false,
-  prepareNewChildrenBeforeUnmountInStack: false,
+  prepareNewChildrenBeforeUnmountInStack: true,
   disableNewFiberFeatures: false,
 };