Exclude modal barrier Semantics in Drawer on Android

[perclasson]

Description

Right now the semantics of the modal barrier for Drawer is visible and accessible for Android even though that is not intended behavior. The way to close the Drawer is via the back button. This change makes sure the dismiss semantics barrier does not show up on Android.

Related Issues

Fixes #39896

Tests

I added the following tests:

"Dismissible ModalBarrier includes button in semantic tree on iOS" test/widgets/drawer_test.dart

"Dismissible ModalBarrier is hidden on Android (back button is used to dismiss)" test/widgets/drawer_test.dart

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Breaking Change

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[HansMuller]

LGTM