Summary:If you're working on an app that needs to support landscape, reloading the app causes the iPhone simulator to reset its orientation to portrait every time the `RCTDevLoadingView` shows up. This is because it sets the root VC to `RCTModalHostViewController`, which currently supports only portrait orientations on iPhone. Changing the root view to a vanilla `UIViewController` fixes the issue.
**Steps to Reproduce**
1. Create a blank RN project: `react-native init RNTest`
2. Open it up and run it.
3. Rotate to landscape `Cmd+Right Arrow`.
4. Reload by pressing `Cmd+R`.
**Expected**
The simulator stays in landscape mode.
**Actual**
The simulator goes back to portrait.
Closes https://github.com/facebook/react-native/pull/6765
Differential Revision: D3127339
Pulled By: javache
fb-gh-sync-id: e2543c90c8d116307dcefa89a417447c1f1a327f
fbshipit-source-id: e2543c90c8d116307dcefa89a417447c1f1a327f
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