Summary:
Currently the RCTActionSheetManager attempts to present itself from the 'rootViewController' of the key window, presenting a modal from a view controller which is already presenting a modal is not allowed on iOS and this fails with the following error appearing in the XCode debugger (but not the chrome debugger):
```
Warning: Attempt to present <UIAlertController: 0x7fca85b14f40> on <UINavigationController: 0x7fca84812000> whose view is not in the window hierarchy!
```
This change recursively looks through modally presented view controllers until it finds the top one and then uses that to present the action sheet.
Closes https://github.com/facebook/react-native/pull/5263
Reviewed By: svcscm
Differential Revision: D2823201
Pulled By: nicklockwood
fb-gh-sync-id: aad1ad88115563f633fd9aaea8e27d1d155a6c27
43dcdaffe2