Summary:
The Navigator component inconsistently emits the `willfocus` event.
While it is emitted in `Navigator#resetTo`, `Navigator#push`, `Navigator#pop` and event `Navigator#replaceAtIndex` it is not emitted from `Navigator#immediatelyResetRouteStack`. This leads to surprising inconsistencies when working with these events.
With the PR I suggest to emit the `willfocus` event before resetting the route stack.
Closes https://github.com/facebook/react-native/pull/10125
Differential Revision: D3931284
fbshipit-source-id: 6e4f45c6d38426bcd0acc8f8c39478524032a03a
95b1fc42b7