Vendor NavigationExperimental, update example to sdk15

[janicduplessis]

https://github.com/facebook/react-native/pull/13066 will remove NavigationExperimental from React Native. This vendors in the latest version with removed haste modules to avoid conflicts. Also update the example to sdk 15.

Tested that it works with or without NavigationExperimental present in React Native.

[chirag04]

@janicduplessis I guess we need on more sync. let's push this given that RN 0.43 is out.

[janicduplessis]

The commit that removes ex-nav should be in 0.44-rc so we should land this soon.

[brentvatne]

merged :O

[janicduplessis]

🎉 Thanks Brent!