Refactors Slider/RangeSlider keyboard navigation for a more intuitive and accessible experience, removing the need to press ENTER to toggle between navigation and value adjustment modes.
With this change:
- Arrow keys (+/-/=/DPAD) always adjust the value of the focused thumb.
- TAB and Shift+TAB always navigate focus between thumbs.
This simplifies event handling by ensuring `activeThumbIdx` stays synchronized with `focusedThumbIdx` during keyboard input.
Also fixes regressions in `SliderTouchTest` by preventing `onFocusChanged` logic (now conditioned on `activeThumbIdx == -1`) from incorrectly overriding thumb selection during touch events.
PiperOrigin-RevId: 816168251
3d8d23dadb