fix: disable screen readers on stacked transparentModal screens for improved accessibility

[theonetheycallneo]

Motivation

Web screen readers can see the components on the previous underlying screens.
I noticed we already do this for iOS and Android via:

accessibilityElementsHidden={!focused}
importantForAccessibility={focused ? 'auto' : 'no-hide-descendants'}

So, simply adding the 3rd aria-hidden to improve web acessibility.

Test plan

1. Create 2 screens, 1 normal and 1 with presentation: transparentModal

• first screen should have 1+ text component(s)
• screen 2 can be anything simple

2. Open the transparentModal screen from the previous screen
3. Enable screen reader/voice over
4. Note: The elements on the 2nd screen are no longer visible by the screen reader

Related:
necolas/react-native-web#2773

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[satya164]

Thanks for the PR. This is part of a larger effort to replace all a11y related props with aria props, which are now supported in React Native so that we can support all platforms with the samecode. However I haven't been able to finalize it: #11848

If you'd like to take over that PR, then feel free. The biggest part left is to test the changes to see if it works, and rebase the PR against latest changes.