Make `SliderInteraction.slideOnly` more consistent in its user feedback

[pogojotz]

Use case

If a Slider with allowedInteraction: SliderInteraction.slideOnly is dragged by its handle, the user feedback includes the activation animation (elevation shadow + overlay). If a user taps on the track, there is no visual indication at all, that the Slider is interacted with, until the user starts dragging and sees the Slider move.

Proposal

We find this behavior inconsistent and suggest to drive the same activation animation, once the track is tapped and a dragging would move the handle.
Looking at the code this would even simplify things and remove a bit of the special treatment of slideOnly. I will prepare a PR shortly.

[dkwingsmt]

Is there a official slider in Material? Have you checked how it behaves there?

[pogojotz]

There is an official slider, but as far as I see it, there is no equivalent interaction to slideOnly defined there.

According to the design document https://docs.flutter.dev/go/permissible-slider-interaction, I pulled from this PR #121483, the slider behaves as suggested in this issue (see the animation under Proposal #1->slideOnTrack). Maybe this detail got lost during implementation. Keep in mind, that the interaction is called slideOnTrack in that document and was renamed during implementation to slideOnly.

[github-actions]

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