Flutter views can gain focus

[tugorez]

I am unsure why the tabindex was removed when semantics were enabled. It seems this change was made without a clear explanation (by me). This PR shouldn't cause any issues as Flutter Views already have a tabindex, we're not adding a new one. This change is necessary because the semantics text strategy refocuses the view on deactivation, requiring the Flutter view to be focusable.

ThIs PR is a requirement to enable https://github.com/flutter/engine/pull/54966.

https://github.com/flutter/flutter/issues/153022

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