[ios] Opening app from recents screen can reopen conversation from previous notification

[gnprice]

This is exactly the same behavior as described in #4758, but on iOS. Filing as a separate issue because the fix will likely be pretty much independent.

Here's the repro steps I just tried, in v27.162 on an iPhone 7 running iOS 12.4.1:

1. Ensure the app isn't running in the background: double-tap the home button to get the recents screen, look for Zulip, and if found swipe it away.
2. Receive a notification.
3. Open the notification. App starts up, and navigates to conversation.
4. Navigate up to the unreads screen, either by swiping from left edge or tapping the "back" button at top left.
5. Tap home button to get to system home screen. App is in background.
6. Double-tap home button to get to recents screen. Zulip app appears, showing the unreads screen we were just on.
7. Tap the Zulip app on recents screen.
   • Expected: Zulip returns to foreground, showing the same screen we were just on.
   • Actual: Zulip returns to foreground, then promptly navigates again to the conversation the notification had been in.

One quirk which I was surprised by, and seems like a clue:
8. Repeat steps 4, 5, 6, 7: navigate up to unreads, tap home, double-tap home to get recents, select Zulip.

• Desired: Zulip returns to foreground, showing the same screen we were just on.
• Expected given the buggy behavior above: Zulip returns to foreground, then promptly navigates again to the conversation the notification had been in.
• Actual: The non-buggy, desired behavior: Zulip returns to foreground, showing the same screen we were just on.

IOW the issue only seems to affect the first time you use the recents screen to reopen the app, not any subsequent times.

[gnprice]

To fix this:

• Probably we should complete Use @react-native-community/push-notifications-ios. #4115 / Start using @react-native-community/push-notification-ios #4163 first, just so we're on current versions of things.
• Then check this still reproduces. I expect it won't change, though -- it doesn't appear that the PushNotificationIOS.getInitialNotification interface has changed.
• Then, it might be enough to add some logic in JS where we invoke that function, remembering if we've already done so once and if so not doing it again.
• Or it might not. Empirical testing will be needed. See our guide for testing notifications changes on iOS.