Fullscreen modal buggy (flickering) when using CustomTabBar

### Bug summary

When using a custom tab bar as by your example, full-screen modals behave inconsistently when no animation is used `(animation: "none")`. Upon navigation, the new screen’s height is initially calculated as SCREEN_HEIGHT - TABBAR_HEIGHT, and shortly after, it adjusts to include the height of the tabbar, causing noticeable flickering.
In other cases, the new screen ends up with a black area at the bottom, matching the height of the tab bar.

Navigating to the modal shortly still shows the tabbar sometimes (for a frame):

Then shortly after renders the full screen as supposed to:

It also happens that it just shows a black area at the bottom part all the time:

### Library version

0.9.2

### Environment info

```shell
⚠️ react-native depends on @react-native-community/cli for cli commands. To fix update your package.json to include:

"devDependencies": {
"@react-native-community/cli": "latest",
}
```

### Steps to reproduce

Navigate to a screen that has the following options:
```
options={{
title: "Modal",
presentation: "fullScreenModal",
animation: "none",
}}
```

### Reproducible sample code

```js
https://github.com/dylancom/native-tabs-modal-bug
```

Start by running the reproducible sample at https://github.com/dylancom/native-tabs-modal-bug and navigating with presentation set to fullScreenModal and animation set to none. Trace the CustomTabBar and modal layout transition that produces the initial tab-bar height, then verify that navigation consistently renders the full-screen modal without flickering or a black area at the bottom.

Scritto dal modello di indicizzazione a partire dal testo della issue.