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