[Experiment] Convert AccessibilityFeatures to enum

[bernaferrari]

Kind of surprising, but part of #40567.

There is a test there that applies to this, and even though I can duplicate the test and fix for SemanticsAction, if this is used as an enum, it can be an enum.

Right? Let's see if tests pass.

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[bernaferrari]

This is confusing, theoretically it could be possible, but the class is weird. It is used as an enum, but its parents (EngineAccessibilityFeaturesBuilder) are used "correctly". Yet, _updateAccessibilityFeatures still expects the value to be passed in a correct way. My test didn't work, I'll duplicate the method in the other branch.