Summary:
To help with migration from direct usages of RCTRootView to RCTSurfaceHostingView, RCTSurfaceHostingProxyRootView is added, which is simply a custom impl of RCTSurfaceHostingView, but will all RCTRootView APIs preserved. This makes it easy to do a drop-in replacement in native callsites:
```
// before:
RCTRootView *rootView = [[RCTRootView alloc] init...];
// after:
RCTRootView *rootView = (RCTRootView *)[[RCTSurfaceHostingProxyRootView alloc] init...];
```
Reviewed By: shergin
Differential Revision: D7141696
fbshipit-source-id: db8c447749eaa896efaa37774a9babef132128eb
Kevin Gozali