fix(material/slider): some screen readers announcing long decimal values (#20721)

It looks like some screen readers announce the value of a slider by calculating the
percentage themselves using the `aria-valuemin`, `aria-valuemax` and `aria-valuenow`.
The problem is that they don't round down the decimals so for a slider between 0 and 1
with a step of 0.1, they end up reading out values like 0.20000068. These changes work
around the issue by setting `aria-valuetext` to the same value that we shown in the thumb
which we truncate ourselves.

Fixes #20719.

Author: crisbeto

src/material/slider/slider.spec.ts

@@ -507,6 +507,16 @@ describe('MatSlider', () => {
       expect(sliderInstance.value).toBe(0.3);
     });
 
+    it('should set the truncated value to the aria-valuetext', () => {
+      fixture.componentInstance.step = 0.1;
+      fixture.detectChanges();
+
+      dispatchSlideEventSequence(sliderNativeElement, 0, 0.333333);
+      fixture.detectChanges();
+
+      expect(sliderNativeElement.getAttribute('aria-valuetext')).toBe('33');
+    });
+
   });
 
   describe('slider with auto ticks', () => {

src/material/slider/slider.ts

@@ -134,6 +134,13 @@ const _MatSliderMixinBase:
     '[attr.aria-valuemax]': 'max',
     '[attr.aria-valuemin]': 'min',
     '[attr.aria-valuenow]': 'value',
+
+    // NVDA and Jaws appear to announce the `aria-valuenow` by calculating its percentage based
+    // on its value between `aria-valuemin` and `aria-valuemax`. Due to how decimals are handled,
+    // it can cause the slider to read out a very long value like 0.20000068 if the current value
+    // is 0.2 with a min of 0 and max of 1. We work around the issue by setting `aria-valuetext`
+    // to the same value that we set on the slider's thumb which will be truncated.
+    '[attr.aria-valuetext]': 'displayValue',
     '[attr.aria-orientation]': 'vertical ? "vertical" : "horizontal"',
     '[class.mat-slider-disabled]': 'disabled',
     '[class.mat-slider-has-ticks]': 'tickInterval',