RNTester Globally Caches Screen Width

[NickGerleman]

Description

Relates to microsoft/react-native-windows#6303

RNTester after being redesigned sets the width of an example paged based on a globally cached width from Dimensions.

https://github.com/facebook/react-native/blob/master/packages/rn-tester/js/components/ExamplePage.js#L25

const ScreenWidth = Dimensions.get('window').width;
import {RNTesterThemeContext} from './RNTesterTheme';

const styles = StyleSheet.create({
...
  examplesContainer: {
    width: ScreenWidth,
    flexGrow: 1,
  },
...
});

Setting the width explicitly isn't needed, but causes a couple of issues. The first is when the viewport is resized, invalidating the globally cached dimensions. Seen below on Windows:

This same issue is likely to be seen if the app supports rotation, or windowing on phone/iPad. It causes some further issues on desktop platforms though, which may be multi-window, (where the Dimensions API is considered unsafe).

React Native version:

0.0.0-9b094ee77 (>= 0.64)

Steps To Reproduce

1. Open RNTester app
2. Open example
3. Resize Viewport

[NickGerleman]

The fix for this is trivial. Want to spend some time soon on some RN core bits (nightly publish, dropped clicks fix, upstreaming this and maybe something else),