ViewPager child height is doubled in ScrollView

# Bug

First of all, thank you for this great package.

ViewPager child's height is doubled (or so) when ViewPager is embedded inside a ScrollView. This affects the dynamic content's UI I am fetching from my API (e.g. I receive content of 300px height and the ViewPager scales to ~600px).

I don't know if this is a bug or normal behaviour, but I do want to know how to work around this issue without any `marginBottom: -100%` non-reliable code.

Here is a sample video from my app. Although, another interesting thing is that it doesn't double on first render, because I present an while the content is downloaded, but it overextends just as I scroll to another page:


## Environment info

`react-native info` output:

```bash
System:
OS: macOS 10.15.4
Shell: 5.7.1 - /bin/zsh
Binaries:
Node: 13.8.0 - /usr/local/bin/node
Yarn: Not Found
npm: 6.14.4 - /usr/local/bin/npm
Watchman: 4.9.0 - /usr/local/bin/watchman
Managers:
CocoaPods: 1.9.1 - /usr/local/bin/pod
SDKs:
iOS SDK:
Platforms: iOS 13.4, DriverKit 19.0, macOS 10.15, tvOS 13.4, watchOS 6.2
Android SDK:
Android NDK: 21.0.6113669
IDEs:
Android Studio: 3.6 AI-192.7142.36.36.6308749
Xcode: 11.4.1/11E503a - /usr/bin/xcodebuild
Languages:
Python: 2.7.16 - /usr/bin/python
npmPackages:
@react-native-community/cli: Not Found
react: 16.13.0 => 16.13.0
react-native: ^0.62.0 => 0.62.0
npmGlobalPackages:
*react-native*: Not Found
```

Library version: 3.3.0

## Steps To Reproduce

1. Embed ViewPager (`flex: 1`) in ScrollView.
2. Add View (check its height with e.g. 'backgroundColor: 'red'`) with content that is bigger in size than the ViewPager (e.g. bigger than the screen).
3. Scroll to next page and see that the ViewPager increased in height way beyond its initial height is extended.

Describe what you expected to happen:

1. The children to not be stretched or to be stretched only as big as the ViewPager (not to overflow its height).

## Reproducible sample code

```js
import ViewPager from '@react-native-community/viewpager'
import React from 'react'
import { ScrollView, View } from 'react-native'

const CHILD_HEIGHT = 300

const ViewPagerHeightBug = (): JSX.Element => (


{Array.from(Array(2)).map((item, index) => (



))}


)

export default ViewPagerHeightBug
```

As you can see in this example, whatever the `CHILD_HEIGHT` we set, the ViewPager somehow doubles.

Inizia con l’esempio ViewPagerHeightBug e il relativo annidamento di ScrollView/ViewPager, quindi analizza il comportamento del layout di ViewPager durante i cambi di pagina. Riproduci il problema con CHILD_HEIGHT impostato su 300 e verifica che il cambio di pagina lasci il pager e l’elemento figlio all’altezza prevista invece di raddoppiarla.

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