Fix notch handling, with SafeAreaView

[borisyankov]

When iPhone X was released, new functionality to iOS was added to indicate safe area - to help with apps handling optional notches, rounded corners or the ...

A good article explaining all this here:
https://medium.com/rosberryapps/ios-safe-area-ca10e919526f

We started using a component react-native-safe-area to handle this.

It isn't working perfectly and we need to fix it.
Testing on iOS X or Simulator with it makes few layout issues obvious.

Better yet, use the now built-in component:
https://facebook.github.io/react-native/docs/safeareaview

[gnprice]

@anirudhjain75 reports that on his Poco F1, Zulip leaves the bottom of the screen black:

From the web, this model appears to have an iPhone X-like notch, so possibly that's causing react-native-safe-area to get confused. Once we've made this change, we'll want to retest and see if that issue is resolved or if it should get separately filed and pursued.

[gnprice]

(The Poco F1 issue is #3068 .)

[gnprice]

It looks like we promptly duplicated this issue as #3066. There's more discussion there; closing in favor of that thread.