To be merged after#12503.

Motivation

iOS modals are presented in a seperate hierarchy. When you open a modal screen and push multiplepresentation: 'card' screens (they are pushed "behind" the modal), the modal screen gets frozen, because it isn'tpreloaded,focused norbelowFocused (pushed screens are above the modal inroutes list). You can't interact with the content of the modal, you can only dismiss it.

In this PR, we prevent freezing modal screens on iOS in order to avoid this problem.

The reasoning behind this change is that you shouldn't show more that 1-2 modal screens at the same time - this is also a recommendation inApple's Human Interface Guidlines:

Let people dismiss a modal view before presenting another one. Allowing multiple modal views to be visible at the same time tends to create visual clutter and can make your app seem scattered and disorganized. People need to remember the context they were in before a modal view appears, so presenting multiple views adds to people’s cognitive load, especially when a modal view hides another one by appearing on top of it. Although an alert can appear on top of all other content — including other modal views — you never want to display more than one alert at the same time.

Test plan

RunTest791 inreact-native-screens' example app.