[material_ui] Mirror the GappedSliderTrackShape secondary track correctly in RTL - #13226

In RTL, GappedSliderTrackShape drew the secondary track from secondaryOffset.dx - trackGap to thumbCenter.dx. That filled the gap next to the thumb, which the main track segments and the LTR secondary track keep empty. The corner radii were also copied from the LTR case instead of being mirrored, so the far end had the small inside corners and the end by the thumb was fully rounded.

This PR draws the RTL secondary track from secondaryOffset.dx to thumbCenter.dx - trackGap, with the fully rounded corners on the far end and the small inside corners next to the thumb. That is the mirror image of the LTR case. The existing visibility check just above already used thumbCenter.dx - trackGap, so it matches the new geometry.

Before and after, LTR on top and RTL below, with Slider(year2023: false, secondaryTrackValue: ...) and trackHeight: 24:

Screenshots demonstration

State	Direction	Screenshot
Before	LTR ➔ ⬅ RTL
After	LTR ➔ ⬅ RTL

I extended the existing Default Slider when year2023 is false test. It now also asserts the exact LTR secondary track, and has a new RTL block with the mirrored inactive, active and secondary tracks, including corner radii. The RTL block fails without the fix.

Fixes flutter/flutter#194279

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